The 20 Best Episodes of 'Star Trek: The Original Series'

20. "The Doomsday Machine"

"Vulcans never bluff." – Spock

After receiving a garbled distress signal that could be a starship's disaster beacon, the U.S.S. Enterprise enters a solar system of decimated planets. Entering a nearby system, they find the same level of destruction and the Enterprise's sister ship: the U.S.S. Constellation, now derelict and commanded by its sole survivor, the prickly Commodore Matt Decker (William Windom). The crew of the Constellation witnessed the destruction of planets by something "right out of hell", as Decker grimly puts it: A gigantic planet killer has been unleashed, an automated weapon designed to digest planetary debris for fuel.

Driven by Sol Kaplan's sonorous score, "The Doomsday Machine" is a suspenseful thriller of an episode. There are distinct echoes of Melville's Ahab in the traumatized Commodore Decker's irrational and obsessive pursuit of the planet killer. There is also the shadow of survivor's guilt. "The commander is responsible for the lives of his crew, and for their deaths," Decker muses. "Well, I should have died with mine." When the deadly device begins pulling the Enterprise into its tractor beam, matters become especially dire. Kirk's plan to destroy the planet killer using a delayed detonation H-bomb device represents everything we have come to know and love about our reckless captain. Strong character development and effective pacing combine in this riveting episode of Star Trek.

19. "Let That Be Your Last Battlefield"

"This will be your final battlefield" – Kirk

When the U.S.S. Enterprise intercepts a stolen Federation shuttlecraft while on a mission to decontaminate a planet plagued by deadly bacteria, a humanoid named Lokai (Lou Antonio) seeking asylum from the war-torn planet Charon is brought aboard. Lokai happens to be split down the middle: black on one side, white on the other. Soon, a similar being shows up from Charon named Bele (Frank Gorshin) who claims to be an officer sent to arrest political traitors. It becomes apparent that Bele and Lokai despise each other because of their racial difference. Tensions over their opposite skin colors drive Bele and Lokai into a war of attrition aboard the ship. In a suspenseful scene with several close-up shots, Bele takes control of the ship while a frustrated Kirk initiates the Enterprise's auto-destruct sequence (later initiated in Star Trek III: The Search for Spock).

Star Trek delivers a powerful message about racial prejudice in this episode that still resonates with viewers today. A strong performance from guest actor Frank Gorshin, best known for his portrayal of the Riddler in the Batman live-action series, truly elevates the episode. Notably, "Let That Be Your Last Battlefield" ends on a bitter note: Bele and Lokai beam down to the surface of their ruined homeworld for a final doomed encounter that is sharply shot and edited. In the end, it doesn't matter who was the dominant or subservient being: both fail to recognize their similarities and pay the ultimate price for their folly.

18. "This Side of Paradise"

"I have a responsibility to this ship, to that man on the bridge. I am what I am, Leila. If there are self-made purgatories, then we all have to live in them. Mine can be no worse than someone else's." – Spock

Was humanity meant to live an Edenic existence? "This Side of Paradise" explores that very question when the U.S.S. Enterprise investigates a colony destroyed by deadly ray beams on the planet Omicron Ceti III. Kirk, Spock, McCoy, Sulu (George Takei), and some redshirts beam down to the planet's surface to discover that Elias Sandoval (Frank Overton) and his colonists are still very much alive and in perfect health, enjoying a pastoral existence off the grid. The colony's botanist, Leila Kalomi (Jill Ireland), reveals harbored feelings for Spock after meeting him years ago, leading to her exposing him to the spores of a flowering plant that cause euphoria and loss of inhibition. The intoxicated Spock then confesses his love for Leila and agrees to live out an idyllic existence with her in the commune.

What follows is a drama of mutiny, temptation, and outright comedy. "I don't know what I can offer against paradise," Kirk frets as he struggles to maintain control over his pacified crew members who have been exposed to the contagion. Kirk's goading of Spock into a physical confrontation makes for an entertaining sequence of both dialogue and action. But it's the bittersweet ending of the episode which leaves a lasting impact. While Kirk wistfully reflects that humanity is not designed for paradise, Spock grapples with the realization that his time with Leila in the colony was the first time he truly experienced happiness. Here, writer D.C. Fontana takes us by surprise, allowing Kirk to take the intellectual lead and developing Spock's half-human emotion, thus bringing well-roundedness to both characters' stories.

17. "The Galileo Seven"

"It may be the last action you'll ever take, Mister Spock, but it was all Human." – Bones

"Totally illogical. There was no chance." – Spock

"That's exactly what I mean." – Bones

In the first Star Trek episode to feature a shuttlecraft, Spock leads a scientific research team from the Enterprise aboard the Galileo on a harrowing mission that begins as an investigation of a mysterious quasar-like formation called the Murasaki phenomenon. Forced to make an emergency landing on Taurus II, a fog-shrouded planet in the middle of the quasar, Spock, and crew face off with a hostile tribe of Taureans, enormous ape-like creatures armed with devastating weapons. Although these violent creatures pose immediate threats to the crew, Spock goes up against his greatest enemy—his own logic—when faced with seemingly impossible decisions of command.

For the first time in the original series, Leonard Nimoy comes center stage and proves that Spock can effectively serve as the driving force of a Star Trek episode. All along the path of his mission, Spock's logic is thwarted by random misfortune. "Strange," he reflects. "Step by step, I've made the correct and logical decisions – and yet two men have died." In the climactic moments of the crew's desperate attempt to escape the planet, Spock makes an illogical gamble to save lives, which he promptly denies as irrational. "Mr. Spock, you're a stubborn man," Kirk quips during some delightful closing banter, which fantastically features that famous Leonard Nimoy eyebrow raise.

16. "The Menagerie" – Parts I & II

"A curious species. They have fantasies they hide even from themselves." – The Keeper

The only two-part episode of Star Trek's initial run, "The Menagerie" harkens back to former star dates when the Enterprise was comprised of a crew much different from the one we know and love. Before William Shatner was cast as James T. Kirk, Star Trek shot a pilot titled "The Cage" starring Jeffrey Hunter as Captain Christopher Pike and a nearly entirely different cast, with the only exception being Spock. At its heart, "The Menagerie" is part clip-show, part Starfleet courtroom drama in which the Enterprise logs are used as evidence, allowing the entire episode of "The Cage" meaningful incorporation into a larger mythos which expands and deepens the Star Trek universe. Featuring Susan Oliver as a green-skinned dancer and the Talosians, a large-headed alien race of telepaths with the power to create illusions indistinguishable from reality, "The Cage" deserves its rightful place in the Star Trek canon.

Some years after the Talosians created the Talosian menagerie, an elaborate test with the goal to foster a servant species to repopulate Talos IV, Spock abducts his former commander, the newly injured and disabled Captain Pike, and locks on course for Talos IV, where the events of "The Cage" took place. According to Starfleet rules, the punishment for traveling to this forbidden planet is death. Spock turns himself in and presents an elaborate story in defense of his actions. "The Menagerie" solidified the role of Christopher Pike, later portrayed by Bruce Greenwood in the J.J. Abrams Star Trek films. The episode successfully expands the Star Trek universe through a clever frame narrative that subverts viewer expectations.

15. "Journey to Babel"

"Well, what do you know? I finally got the last word." – Bones

"One of the points I wanted to make believable in 'Babel' was that both Spock and Sarek were right – as their own convictions applied to themselves – and wrong – as their convictions applied to each other." – D.C. Fontana

During a journey to the planet Babel for a diplomatic conference, one of the visiting ambassadors aboard the Enterprise is murdered. The suspects? A variety of alien dignitaries including Andorian delegate Thelev (William O'Connell) and Vulcan ambassador Sarek, who also happens to be Spock's estranged father (adeptly played by Mark Lenard, who originally played a Romulan commander in the episode "Balance of Terror"). Memorable for introducing Spock's parents, Vulcan Sarek and human Amanda (Jane Wyatt), this episode's philosophical intrigue more than holds up with time.

A mysterious D.C. Fontana-penned plot coupled with a procession of fascinating alien specimens, this adventurous episode is ultimately an exploration of Spock's family dynamic and a journey into what it means to be half-human and half-Vulcan, torn between two drastically different cultures and ideological perspectives. The rift between Spock and Sarek is gracefully handled, with Amanda positioned somewhere in the middle. "Journey to Babel" masterfully handles the implications of placing Spock's hybridity between the parental extremes of the Vulcan and human psyches while introducing a father-son relationship that became a staple of the Star Trek franchise.

14. "Shore Leave"

"The more complex the mind, the greater the need for the simplicity of play." – Kirk

In an episode that was largely rewritten while it was shot, the crew of the Enterprise takes much-needed shore leave on a planet resembling Earth, where figments of their imagination (and ghosts from their past) become reality. After beaming down to the planet's surface, Kirk, McCoy, Sulu, and Yeoman Barrows (Emily Banks) witness absurd fantasies materialize into reality, including the White Rabbit from Alice in Wonderland, an old .38 police special, Don Juan, a Japanese fighter plane, a fairytale princess, and a samurai warrior (among others). It becomes clear as the episode moves along that the crew's thoughts, with all their pleasures and pains, are coming to life.

Eventually, the planet's caretaker appears and confirms that his people constructed the planet as a playground where anything one can think is instantly manufactured—only for the moment. The caretaker informs Kirk that his crew is not yet ready to understand his people, but permits them to stay and enjoy their shore leave. The episode is hilarious, yet not without its tender moments. Kirk's rekindled love for his past flame Ruth (Shirley Bonne) is a bright spot, as is his prolonged fistfight (notable for its blatant use of stunt doubles) with Finnegan (Bruce Mars), a flamboyant jokester of an Irishman who tormented Kirk during his first year at Starfleet Academy. "After all these years, I did enjoy it," Kirk remarks after relishing an epic beatdown on Finnegan out in the planet's desert landscape.

13. "Mirror, Mirror"

"Jim, I think I liked him with a beard better. It gave him character. Of course, almost any change would be a distinct improvement." – Bones

A violent ion storm rips through space, while a landing party from the Enterprise comprised of Kirk, Scotty (James Doohan), McCoy, and Uhura (Nichelle Nichols) fails to earn the right to mine dilithium on the homeworld of the pacifist Halkan people. A transporter malfunction quickly sends Kirk, McCoy, Scottie, and Uhura to a parallel universe where the Federation is the evil Terran Empire, Kirk is a tyrant, and Spock is a sinister, goateed henchman dubbed "Spock Prime".

A definitive science-fiction depiction of the implications of parallel realities, "Mirror, Mirror" is as famous for its commentary on fascism and the duality of good and evil as it is for repurposing the goatee to signify the now household "evil doppelganger" trope. The episode has endured because of the strength of its concept, its exploration into terror as necessity for upholding fascist regimes, its comic seduction scene between Uhura and Sulu, and Spock's keen observation of his totalitarian counterparts as "...brutal, savage, unprincipled, uncivilized, treacherous; in every way splendid examples of Homo sapiens: the very flower of Humanity." The alternate reality plot was re-explored in several episodes of Star Trek: Deep Space Nine.

12. "The Trouble with Tribbles"

"Do you know what you get if you feed a tribble too much?"

"A fat tribble."

"No. You get a bunch of hungry little tribbles." – Bones and Kirk

In one of Star Trek's most enduring episodes, drama unfolds on the space station K-7 when the Enterprise suspects that the Klingons will disrupt a grain delivery to an Earth colony. All is complicated when Uhura purchases a Tribble, a small, furry, purring creature resembling a hairball (somewhat like a less talkative Furbie). Bones soon discovers that Tribbles are born pregnant. It's too late: Federation politics take a backseat when they begin to multiply exponentially and the ship has to deal with its surplus of uninvited critters.

To commemorate the 30th anniversary of the franchise, Star Trek: Deep Space Nine returned to "The Trouble with Tribbles" with an episode called "Trials and Tribble-ations", where the crew of DS9 travel back in time to prevent a plot to assassinate Kirk. The fan-favorite was nominated for three Emmy awards and is notable for its digital insertion of Deep Space Nine actors into the background of the original "Tribbles" episode, expanding the Star Trek universe while paying homage to the legacy of the original series. A Tribble also plays an important role in the film Star Trek Into Darkness.

11. "The Naked Time"

"This vessel. I give, she takes. She won't permit me my life. I've got to live hers." – Kirk

George Takei's favorite episode of Star Trek. When Spock and a red shirt named Tormolen (Stewart Moss) beam down to the planet Psi 2000 in environmental suits to investigate a frozen laboratory full of dead scientists, they become exposed to a dangerous contagion that strips humans of their inhibitions. After beaming back aboard, Tormolen ends up killing himself in a fit of madness. Another crewman named Riley (Bruce Hyde) suddenly begins acting especially Irish. Most famously, Sulu begins to parade around the ship with a sword before being subdued by Spock in the first appearance of the "Vulcan nerve pinch." Riley ends up taking over the engineering room, shutting off the ship's engines, and serenading the crew with an Irish ditty, "I'll Take You Home Again Kathleen." Suddenly the Enterprise is in a state of total chaos.

Eventually, Kirk is infected by the euphoric contagion. Fighting back his desire for Yeoman Rand (Grace Lee Whitney), he must regain his self-awareness through sheer force of will. In a thrilling escape from the planet's orbit, Scotty and Spock discover that they have sent the Enterprise back three days in time. This is Star Trek at its comedic best, but the episode is also cherished for Leonard Nimoy's performance as a tormented Spock. "I could never tell her I loved her," Spock laments about his mother. According to his autobiography, Nimoy told writer John D.F. Black that he had the following in mind for that particular scene: "It's about emotion versus logic, love versus mathematics, grief versus pi-r-squared." Based on Nimoy's suggestions, Black went back and rewrote the scene, transforming it from a simple sight gag into one of Nimoy's finest acting achievements on Star Trek.

10. "Amok Time"

"What thee are about to see comes down from the time of the beginning, without change. This is the Vulcan heart. This is the Vulcan soul. This is our way." – T'Pau

"After a time, you may find that having is not so pleasing a thing after all as wanting. It is not logical, but it is often true." – Spock

The premiere episode of Star Trek's second season takes us to the Vulcan homeworld, where mysteries of the Vulcan way of life are revealed. When Spock suddenly suffers from pon farr, a Vulcan mating cycle occurring every seven years, he must return home to take a mate or face imminent death. On Vulcan, Kirk and McCoy are invited to attend Spock's marriage to his betrothed, T'Pring (Arlene Martel), and to witness the Vulcan marriage ceremony led by T'Pau ( Celia Lovsky), the Vulcan master of ceremonies. Shockingly, Spock's would-be wife invokes kal-if-fee, her right to have Spock fight for her. What's more, she picks Kirk as her unwilling champion!

"Amok Time" boasts a thrilling gladiatorial battle between Spock and Kirk while revealing the mystical secrets of Vulcan rituals. The episode features one of Leonard Nimoy's very finest performances as Spock and brings Spock and Kirk closer together than ever before. Nimoy's performance is nuanced and moving, culminating in a scene in which Spock believes he has killed not only his captain but his dearest friend. In addition to establishing the concept of pon farr, which later appears in Star Trek: The Search for Spock, the episode introduced the Vulcan salute and the immortal phrase: "Live long and prosper."

9. "The Corbomite Maneuver"

"Not chess, Mr. Spock. Poker! Do you know the game?" – Kirk

While developing star maps of a distant region of space, the Enterprise is confronted by a powerful spherical alien ship called the Fasarius, the flagship of the First Federation commanded by a ruthless, sophisticated being named Balok. When Balok threatens to destroy the ship, Kirk comes up with a cunning bluff to convince the alien that the Enterprise is harboring a deadly substance called corbomite capable of destroying both ships.

"The Corbomite Maneuver" is arguably the first Star Trek episode to tout Kirk's daring during a face-off with another ship in space. It's also the most classic example of the importance of bluffing in the original series. Kirk bends the rules for the greater good and turns a potentially fatal situation into a victory, laying the foundation for similar events in Star Trek: The Wrath of Khan and Star Trek: The Search for Spock. Through the power of imagination, a better outcome than violence is achieved. Kirk's bold move to break with Starfleet regulations and make the wrong decision at the right time solidifies his place as a leader who thinks outside the box to protect his crew. Another way "The Corbomite Maneuver" surprises in its reveal that appearances can be deceiving in a final act featuring Clint Howard in an early role.

8. "The Enemy Within"

"He's like an animal. A thoughtless, brutal animal. And yet it's me. Me!" – Kirk

Star Trek takes on Dr. Jekyll and Mister Hyde in a sci-fi context in an episode that delivers a classic performance by William Shatner. During a survey of the planet Alpha 177, a technician is exposed to a magnetic ore that alters the function of the Enterprise's transporter. When Kirk beams aboard the ship, the transporter splits him into two beings: one good, one evil. After the new animalistic Kirk attempts to force himself on Yeoman Rand, Spock deduces that there is an imposter aboard the Enterprise.

Meanwhile, Kirk experiences problems with decision-making, while Sulu and three other crew members remain stranded on the planet's surface. McCoy informs Kirk that all humans have a dark side to them and that his strength of command lies in his negative self. After a confrontation on the bridge where it's unclear which Kirk is which, Evil Kirk eventually breaks down under the strain of the circumstances around him, crying out, "I want to live!" Kirk is materialized back into one person and the crew members below are beamed aboard the ship.

It's difficult to take one's eyes off William Shatner in this episode. Moving from hesitance to lustfulness, from violence to sorrow, Shatner delivers his most nuanced, albeit over-the-top performance in the history of Star Trek. The episode is also celebrated for its insightful look into the conflicting dualities of human nature. By the way, did I mention that a certain adorable alien canine gets divided into two adorable alien canines? This one is a must-watch.

7. "The Man Trap"

"Something wrong, captain?" - Spock

"I was thinking about the buffalo, Mister Spock." – Kirk

The first episode of Star Trek to air on television, "The Man Trap" develops whimsical chemistry between our favorite crewmembers and features both a creepy monster-of-the-week and a gorgeous planetary backdrop. Kirk, McCoy, and a red shirt (Michael Zaslow) beam down to the planet M-113 to provide medical supplies to Dr. Robert Crater (Alfred Ryder) and his wife Nancy (Jeanne Bal), who just happens to be an old flame of McCoy's. Mysteriously, each crew member perceives Nancy as a different woman from his past. Also odd is Dr. Crater's intense desire for salt tablets. When the red shirt is found dead nearby due to a sudden lack of salt in his body, it becomes evident that McCoy's former lover is not what she appears to be.

The wise-cracking between Kirk and McCoy about the past romance between Bones and Nancy immediately establishes Kirk as something more spontaneous than an authoritarian ship captain and Bones as a charmingly ornery country doctor. Spock's logic-based personality is also fleshed out here, notably through a flirtatious scene between the Vulcan and Uhura, which may have inspired the romance between Uhura and Spock in the J.J. Abrams Trek films. Also showcased is Sulu's interest in botany and Grace Lee Whitney's beehive-touting Yeoman Janice Rand. The episode's Salt Vampire is happily not simply a one-dimensional, clichéd monster, but rather an intelligent being, the last of its kind, following its natural survival instincts. The tragic existence of the Salt Vampire is sorrowfully compared to that of the buffalo.

6. "Balance of Terror"

"You and I are of a kind. In a different reality, I could have called you friend." – Romulan commander

The U.S.S. Enterprise battles a Romulan ship suspected of destroying outposts in the Neutral Zone in a thrilling cat-and-mouse space opera based on the 1957 movie The Enemy Below, with the Enterprise taking on the role of the American destroyer and the Romulan Bird-of-Prey with its cloaking device playing the role of the submarine. Viewers learn that since two-way visual communications did not exist during the Earth-Romulan war of the recent past, Romulans and humans have never seen one another. Not only does the Enterprise have to confront a brilliant tactician, but it must also confront its own bigotry. When it's revealed that the Romulan commander, brilliantly played by Marc Lenard (who later assumes the role of Spock's father Sarek) resembles a Vulcan, Lieutenant Stiles (Paul Comi) begins to suspect that Spock is a Romulan spy.

Following the Romulan ship into a comet's tail, the Enterprise and the Romulan ship exchange plasma torpedos and phaser fire. After Spock saves Stiles and the ship itself, severely damaging the Romulan vessel, Kirk and the Romulan commander share a moving scene in which the Romulan tells Kirk he admires him and that they could have been friends under different circumstances. Rather than accepting capture, the Romulan commander chooses to destroy his own craft and himself with it. One of the strongest episodes of Star Trek, "Balance of Terror" introduces a new alien race in the Romulans in a bold entry in the larger Trek mythos that showcases Kirk's ability to outmaneuver his enemy under most dire conditions.

5. "Arena"

I shall be merciful and quick." – The Gorn

Captain Kirk squares off against the Gorn captain in a battle of both might and wits on a desolate planet in one of the most enduring Star Trek episodes of all time. After the Cestus III Outpost is obliterated by a species of aliens, the Enterprise engages the aggressor ship. Suddenly, the Enterprise is contacted by a species of aliens called the Metrons (voiced by Vic Perrin) who zealously guard their sector of space against outsiders. They announce they will pit the respective captains of both ships against one another in a "trial by combat", a one-on-one battle to the death, with the ship on the losing side to face destruction and the other ship free to leave. The captains are beamed down to a rocky desert planet with various resources that might be used to fight one another. On the Enterprise, the crew helplessly looks on while Kirk engages the reptilian Gorn (voiced by Ted Cassidy) in mortal combat.

In an episode centered on themes of survival and mercy, Kirk must learn to understand his enemy's thought process and overcome the fact that he is wholly outmatched by a physically stronger opponent. While "Arena" is a thrilling and ethically interesting romp in space, it will forever be remembered for its amazing Gorn costume. Charming levels of '60s science fiction cheese elevate "Arena" into the upper echelon of Star Trek lore.

4. "Where No Man Has Gone Before"

"Do you like what you see? Absolute power corrupting absolutely?" – Kirk

After investigating what happened to the U.S.S. Valiant at the edge of our galaxy, the Enterprise encounters a magnetic space storm that grants Lieutenant Commander Gary Mitchell (Gary Lockwood) dangerous, godlike powers and extrasensory perception. Though the show's second pilot was rejected by NBC, the episode title "Where No Man Has Gone Before" was adopted as the closing phrase of the opening credits voice-over by William Shatner and has gone on to shape both science fiction and pop culture as we know it. Featuring breathtaking action sequences and fine character acting by Lockwood and Sally Kellerman as Dr. Elizabeth Dehner, "Where No Man Has Gone Before" is a notoriously underrated episode that offers a critical glimpse into humanity's struggle for power and the corruption it breeds.

When Mitchell, a former friend of Kirk's from Starfleet Academy, unleashes his godlike telepathic and telekinetic powers on the crew of the Enterprise, Spock suggests that he should be killed. Kirk angrily disagrees, marooning him on a lithium-cracking facility on a remote planet. Yet Mitchell seduces Dr. Dehner and gives her similar deadly powers. The episode's final showdown in which Kirk appeals to Dehner's humanity anticipates a similar pivotal scene from Star Wars: The Return of the Jedi. Ultimately, Kirk's recognition that Mitchell "didn't ask for what happened to him" begins a tradition of sympathetic villains on Star Trek.

Fun fact: Kirk's middle initial appears as "R" on the gravestone Mitchell creates for Kirk. Subsequent episodes use "James T. Kirk," and eventually Kirk's middle name is revealed to be Tiberius on Star Trek: The Animated Series.

3. "Space Seed"

"It is better to reign in hell than serve in heaven." – Kirk, quoting John Milton's Paradise Lost

"Space Seed" introduced Star Trek's most popular antagonist: the genetically enhanced superman from the 20th century, Khan Noonien Singh, masterfully portrayed by Ricardo Montalbán. The Enterprise comes across a long-lost Earth vessel, the Botany Bay (named after the Australian penal colony), containing a cryogenically frozen Khan and his original crew. After manipulating historian Lieutenant Marla McGivers (Madlyn Rhue) into joining his crusade, Khan and his superhuman soldiers take command of the Enterprise. A classic face-off between Kirk and Khan ensues.

Khan serves as the perfect villain for Kirk to take on, a mentally and physically superior being who threatens his command and the safety of his crew. Not only that, but Khan is a complex character with an interesting backstory involving eugenics wars on Earth in the 1990s. Kirk's undeniable admiration for Khan raises some interesting moral questions and sparks philosophical dialogue that still resonates today. Khan eventually accepts being marooned on the uninhabited planet Ceti Alpha V over a court-martial, referencing Satan from Milton's Paradise Lost in his decision.

Although "Space Seed" is Khan's only appearance in the original series, Montalbán takes up the mantle once more in Nicholas Meyer's Star Trek: The Wrath of Khan, considered by many to be the very best of the Star Trek films. Benedict Cumberbatch would go on to play an alternate reality version of the character in J.J. Abrams' Star Trek Into Darkness. Put simply, Khan is the greatest Trek villain.

2. "The City on the Edge of Forever"

"The men that reach out into space will be able to find ways to feed the hungry millions of the world and to cure their diseases. They will be able to find a way to give each man hope and a common future… and those are the days worth living for." – Edith Keeler

Penned by the late legendary author Harlan Ellison, "The City on the Edge of Forever" is a perfect Star Trek episode, demonstrating elegant pathos while exploring the best of what science fiction can achieve. Suffering from an accidental overdose that brings on a frenzy, McCoy beams down to an ancient alien world where a mystical gateway dubbed "The Guardian of Forever" sends him back in time to 1930s Earth. There, he saves a woman's life, unwittingly altering the course of time and erasing the Enterprise and the Federation from history. Trapped in the limbo of non-existence, Kirk and Spock travel back in time in a desperate attempt to rectify the disturbance and correct the course of history.

Kirk and Spock travel to 1930s New York City where they meet Edith Keeler (Joan Collins), a woman who has dedicated her life to helping those in need. While Spock works on building a computer to access information on his tricorder, Kirk and Keeler develop a romantic relationship. Then comes some devastating news: In order to fix the time alteration, Edith Keeler must die. Spock explains to Kirk that Keeler will go on to organize a pacifist movement in the U.S. that will delay U.S. involvement in World War II. Keeler must now be sacrificed to achieve the very peace she fought so hard to attain. Ellison's writing combines with electric chemistry between Shatner and Collins to deliver a bittersweet episode of Star Trek.

1. "The Devil in the Dark"

"There's nothing more dangerous than a wounded animal." – Kirk

"The Horta has a very logical mind. And after close association with Humans, I find that curiously refreshing." – Spock

Of "The Devil in the Dark", Leonard Nimoy once noted, "I thought it was a wonderful episode about the fear of the unknown, how we fear and even hate something that we don't know anything about, learn who your enemy is, and it's not, maybe then it's no longer your enemy."

In the episode, which features one of the most elaborate set designs of the original series, The Enterprise crew responds to a distress call from a mining colony on Janus VI where workers are being reduced to ash by a rocklike creature excreting molten lava. When Kirk and Spock arrive, there is no sign of the creature—only traces of its victims—and they begin to ponder how a carbon-based life form could exist in such a cavernous environment. This is Star Trek coming face to face with the unknown at its purest, most satisfying, most speculative level. The creature in question is the Horta—a species that we soon discover is more logical than we might first imagine.

Although silicon-based and not carbon-based, like all lifeforms, the Horta simply does what it must to survive. Spock establishes a Vulcan mind meld with the creature in a mesmerizing scene that reveals the creature's fear and its survival instincts. It turns out that the silicon spheres the workers on the planet have been mining are Horta eggs. Kirk convinces the miners to allow the Horta to live among them and dig for mineral deposits, envisioning a world of vastly different lifeforms working towards the common goals of survival and prosperity. Although we have yet to learn from the lessons of "The Devil in the Dark", the episode inspires hope in the form of a harmonious vision of life, conjuring deep emotions that only the finest episodes of Star Trek can inspire.