How did a low-budget sci-fi show from the 1960s, one almost canceled after its second season, become one of the most important franchises of all time? Answers range from a rabid fan base that kept Star Trek alive after its second season to a network putting support behind a recognizable IP. But the truth is far more simple: Star Trek: The Original Series is really, really good.Like fellow 60s greats Doctor Who and The Twilight Zone, Star Trek used science fiction to comment on social changes and to wrestle with philosophical ideas. The show overcame its budget limitations to tell genuinely thrilling stories, and gathered an incredible cast, grounded by the endlessly charismatic William Shatner as James T. Kirk and the inscrutable Leonard Nimoy as Spock.Honestly, most of season one and two are good to great (also, theres a third season!) but these 20 episodes best demonstrate how Star Trek became the cultural force it is today.20. The Return of the Archons (Season 1, Episode 21)In this era of endless dystopian fiction, including the Purge series that drew inspiration from this episode, its hard not to roll your eyes at the premise of The Return of the Archons. Written by Boris Sobelman from a story by Gene Roddenberry (who usually revised/rewrote episodes) and directed by Joseph Pevney, The Return of the Archons sends the Enterprise to the seemingly utopian planet Beta III. Within the revery, however, lurks a dictator called Landru, who instates a period of lawlessness called The Red Hour.Despite its goofy premise, The Return of the Archons gets at a fundamental question about the lengths to which well go to participate in a society, even when theyre morally questionable lengths. The episode has an allegorical force that throws into question the very idea of utopia, which had already become a key aspect of Star Trek.19. A Piece of the Action (Season 2, Episode 17)To stretch budgets as much as possible, Star Trek would sometimes reuse sets from other shows, which didnt always fit the series sci-fi aesthetic. The series would explain the discrepancies by describing the planet as modeled on Earth culture, a hand wave that might annoy people if the episodes werent so much fun.Case in point, A Piece of the Action, written by David P. Harmon and Gene L. Coon and directed by James Komack. While searching for a long-lost ship, Kirk and Spock visit the planet Sigma Iotia II, which resembles 1930s Chicago. Before you can say You dirty rat, Kirk and Spock are in pinstripes and fedoras, matching wits with mob bosses. A Piece of the Action doesnt have the same high-minded ideals as many episodes on this list, but it does give Shatner and Nimoy the opportunity to show off their comedic chops.18. The Enterprise Incident (Season 3, Episode 2)Every Trekkie knows that TOSs third season pales in comparison to the first two. Still, there are a few gems in there, including the thrilling heist episode The Enterprise Incident, directed by John Meredyth Lucas and written by regular D.C. Fontana. In The Enterprise Incident, Kirk matches wits with the Romulan Star Empire on a secret Federation mission to steal a cloaking device (no Section 31 necessary).Its always fun to see Kirk play figurative three-dimensional chess with both his enemies and his crew. But the real pleasure of The Enterprise Incident comes from watching Spock deal with the clever Romulan Commander, played with menace and allure by Joanne Linville. The interactions further develop the connection between the Romulans and the Vulcans, while shining new light on Spocks internal conflict.17. The Galileo Seven (Season 1, Episode 16)We know were in for something special during the cold open to The Galileo Seven, written by Oliver Crawford and S. Bar-David and directed by Robert Gist, when a striking overhead shot shows the bridge from a new perspective. Even better is the introduction of the shuttlecraft that Spock and a small crew use to investigate a quasar, which will become a key part of Star Trek technology.External qualities aside, The Galileo Seven presents an excellent test of Spocks morals and of larger Federation ideals. Spocks logical approach to dealing with the giants his team encounters on an away mission results in the death of some crew people and conflicts with others. Add in some patented Scotty engineering, and we get a nuanced look at different skills and personalities working together.Join our mailing listGet the best of Den of Geek delivered right to your inbox!16. A Taste of Armageddon (Season 1, Episode 23)Sometimes, hanging a lampshade on an absurd premise can undercut a story. Sometimes, it enhances the storys moral force, which is certainly the case with A Taste of Armageddon. Written by Coon and Robert Hamner and directed by Pevney, A Taste of Armageddon takes place on Eminiar VII, a planet constantly in a state of civil war. Instead of conducting their battles with weapons, their citizens run computer simulations to determine the outcome of battles. Citizens of the losing side willingly enter disintegration chambers to pay the cost of their defeat.Well, they do until the Enterprise gets destroyed during a fight, and Kirk refuses to sacrifice his crew. Kirks utter incredulity at the practice, matched only by the kindness and sincerity with which leader Anan 7 (David Opatoshu) defends his peoples methods, preserves the episodes allegory. Thanks to these performances, our own real-world wars look no more reasonable than the computer games Kirk detests.15. The Enemy Within (Season 1, Episode 5)Yes, William Shatner overacts. If that bothers you, then abandon trying to watch TOS at all. But if Shatners dramatic approach works as well for you as it does most Trekkies, then The Enemy Within is a treat. Written by sci-fi legend Richard Matheson and directed by Leo Penn, The Enemy Within introduces one of Star Treks most reliable plot devices, a transporter malfunction. In this case, the malfunction creates two Kirks, one weak and passive, the other aggressive and decisive.Too often, commenters distinguish the two Kirks on moral grounds, calling the passive one good and the aggressive one bad. But while the latter does, you know, attack Yeoman Rand, the episode really presents them as different parts of what makes Kirk the man he is and therefore not subject to such simple binaries. By eschewing these moral binaries, The Enemy Within establishes Kirk as a far more complex character than those often found on 1960s television, and gives Shatner space to give a couple of big, beautiful performances.14. Errand of Mercy (Season 1, Episode 26)The Klingons evolved drastically over the years, beginning as arrogant and manipulative aliens, portrayed by white guys with bronzer and fake mustaches, to the warrior class with ridged foreheads that well meet later on. Those haughty original Klingons made their debut in Errand of Mercy, led by the delightfully arch Kor (John Colicos).However, discussions of Klingon evolution too often forget how good the aliens first appearance really is. Written by Coon and directed by John Newland, Errand of Mercy, pits Kirk and Kor against one another, as the Federation and the Klingons try to get residents of the planet Organia to align with their respective sides. What follows is one of TOSs better anti-war episodes, a call for peace and understanding that goes even further than the ideal vision of the Federation.13. The Tholian Web (Season 3, Episode 9)The Tholian Web begins with a classic sci-fi mystery, in which the Enterprise discovers the missing starship Defiant, filled with the bodies of crew people who apparently killed one another. Worse, the extreme anger came from the Defiant getting trapped in an interphrase between dimensions, a shift that occurs with Kirk still on the ship. Even worse, aliens called Tholians see the arrival of the Enterprise as an act of aggression, and seek to attack the ship while Spock and McCoy work to retrieve the Captain.By season three, the crew had fully developed its rapport with one another, and thats particularly true of the primary trio of Kirk, Spock, and McCoy. That familiarity drives The Tholian Web, as Spock and McCoys established relationship gives weight to their search for Kirk, while each member of the crew plays their part to deal with the Tholians. In short, The Tholian Web finds the Enterprise at peak capability, with every member of its crew doing their jobs and doing them well.12. The Doomsday Machine (Season 2, Episode 6)For as much as Star Treks future gets described as utopian, its clear that Starfleet still has lots of problems. Case in point, The Doomsday Machine, written by Norman Spinrad and directed by Marc Daniels. Answering the call of the USS Constellation and rescuing the sole survivor Commodore Decker (William Windom), Kirk and crew learns of a gargantuan planet destroyera destroyer that soon threatens the Enterprise.Much of The Doomsday Machine deals with nuts and bolts sci-fi goodness, as Scotty and co. try to repair the Constellation while the Enterprise finds a way to escape. However, the drama comes from Deckers struggle with survivors guilt after the planet destroyer robbed him of purpose. Windom captures the mans despair and desperation, making for a memorable episode, one that leads to the excellent Next Generation novel Vendetta.11. Space Seed (Season 1, Episode 22)It would be understandable to describe Space Seed as a prequel to Star Trek II: The Wrath of Khan, given the way that film looms large in the franchises history. But theres a reason that Nicholas Meyer chose Khan as the TOS villain to bring back for a movie. Its because Space Seed is a really, really good episode of Star Trek.In Space Seed, the Enterprise recovers the USS Botany Bay and releases from suspended animation one of the people inside, the one-time warlord Khan Noonien Singh (Ricardo Montalbn). Immediately, Khan proves himself a threat, thanks to Montalbns incredibly charismatic take on a man who asserts his power in every situation, even charming the Enterprises historian (thats a job?) Lt.Marla McGivers (Madlyn Rhue). So compelling is Montalbns performance and the story by Carey Wilber and Coon that Khan would be a Star Trek great, even if Wrath of Khan never happened.10. Amok Time (Season 2, Episode 1)Like Space Seed, Amok Time often gets diminished to a one-sentence synopsis: the one where Spock gets horny. However, the mating drive known as pon farr is just one of the many aspects of Vulcaln lore that writer Theodore Sturgeon and director Joseph Pevney introduce in Amok Time.Driven mad by his need to mate, Spock returns to Vulcan to reunite with his betrothed TPring (Arlene Martel), only to discover that she has chosen another, which leads to ceremonial conflict. Although Amok Time leans hard into its pulpy depiction of the planet and its people, all involved play it straight, including Celia Lovsky as the matriarch TPau. Thanks to their dedication, Amok Time ends with Vulcan as a compelling place, giving the franchise a foundation for hundreds of interesting stories.9. Mirror, Mirror (Season 2, Episode 4)Like Tremaine and Harry Mudd, the Mirror Universe is a highly divisive TOS concept. Theres no in-between. You must either embrace the over-the-top nature of the tropes or you must skip the episodes altogether. Of the three concepts, the Mirror Universe wins out, thanks to the episode that introduced it, Mirror, Mirror by writer Jerome Bixby and director Marc Daniels.Much has been said about the central idea of Kirk, McCoy, and Uhura arriving in a universe filled with evil doppelgngers, including Spock with a goatee. Equally compelling, however, is the frame narrative around the Mirror Universe adventure, in which the Federation tries to broker a deal for Dilithium crystals from the Halkans. The Halkans refuse on the grounds that the Federation could use the crystals for war, a position supported by the Mirror Universe adventure, despite the episodes conclusion, making the organization more complex than often realized.8. The Corbomite Maneuver (Season 1, Episode 10)Like its contemporary Doctor Who, Star Trek understood its goofy looking aliens were part of the appeal, not a distraction. The Corbomite Maneuver from writer Jerry Sohl and directer Joseph Sargent features a humdinger of a weird alien in Balok, the figure who shows up on the Enterprise view screen as a representative of the First Federation. Of course, as well learn, that figure is just a puppet for the real Balok, a childlike entity portrayed by Clint Howard (voice dubbed by Walker Edmiston).In addition to the cool aliens, The Corbomite Manuever stands out for its demonstration of Kirk as a tactical master. As much as pop culture tends to remember Kirk as a brash womanizer (see Zap Brannigan from Futurama), the actual series depicts him as far more interested in matching wits than wooing ladies. Kirks cunning is on full display (as are Shatners pecs at the start of the episode, which does earn a Kif sigh) in The Corbomite Manuever.7. Arena (Season 1, Episode 18)If an intergalactic godlike being arrived and erased from existence all but one episode of Star Trek from existence, Arena might be the one to save. Obviously, Arena isnt the all-time best episode, but it does have most of Star Treks defining qualities. Weve got a cool rubber alien, weve got Kirk getting his shirt ripped up, and weve got, yes, a godlike being.However, as much as culture remembers Arena as the one where Kirk does the two-handed chop on the Gorn, the episode is more about peace and understanding than it is hand-to-hand combat. Writer Coon (working from a story by Fredric Brown) and director Pevney treat the face-off as a chance for the combatants to embrace their better instincts, to seek connection and communication, even in the face of an unknown danger.6. The Menagerie (Season 1, Episodes 11 and 12)As already discussed, Star Trek had a genius for reusing materials, but The Menagerie exceeds even the highest expectations. Instead of throwing out Where No Man Has Gone Before, the original pilot episode featuring Jeffrey Hunter as Captain Christopher Pike and Nimoy as a smiling Spock, Roddenberry reused the footage to add depth to the Vulcans story for The Menagerie.In The Menagerie, Spock comes to the rescue to his former Captain Pike, his active mind now trapped in an immobile body after a heroic sacrifice. Spock wants to take Pike back to the forbidden planet of Talos IV, resulting in a Starfleet court martial. In between reused footage of The Cage, showing Pikes initial meeting with the Talosians, The Menagerie uses a trial format to show Kirk and Spock at their best, cementing the bond that will become a highlight of the franchise.5. The Trouble With Tribbles (Season 2, Episode 15)Not everyone loves it when Trek gets goofy. But even those who hate Bride of Chaotica! or The Elysian Kingdom have to at least chuckle at The Trouble With Tribbles, written by David Gerrold and directed by Pevney. The episode introduces the infamous rapidly reproducing furballs, which help uncover a secret Klingon plotbut not before annoying everyone first.As is always the case with silly Trek, The Trouble With Tribbles wouldnt work without an incredibly game cast. The famous shot of the creatures dropping on Kirk gets a laugh because of Shatners reaction, somehow overdoing it even when standing still. The mess hall fight that breaks out between Starfleets and Klingons works because of the delicious haughtiness by the latters Captain, Koloth (William Campbell). Even if not every Trekkie loves episodes like The Trouble With Tribbles, everyone on screen seems to be having fun.4. Journey to Babel (Season 2, Episode 10)The joy of Star Treks vision of the future isnt limited to seeing people of different races and genders working together on a starship. Its also the steadfast belief that even aliens can learn to co-exist. Although those qualities become more pronounced in TNG and beyond, they have their roots in TOS, especially Journey to Babel, written by Fontana and directed by Pevney. The Journey to Babel features a Federation conference on the planet Babel, where the delegates discuss the potential inclusion of a new member planet.As its title suggests, Journey to Babel is all about differences and communication. The episode introduces the Tellarites and Andorians, two of the more important Trek species, as well as Mark Lenard and Jane Wyatt as Spocks parents Sarek and Amanda. Add in some quality McCoy grumpiness, and youve got everything great about Star Trek.3. The Devil in the Dark (Season 1, Episode 25)Of course, Star Trek puts an emphasis on logic and clear thinking. But to be a show about exploration and discovery, to be the sort of show that can feature an episode like Journey to Babel, it must fundamentally be about empathy and understanding. And few episodes demonstrate those qualities like The Devil in the Dark, the first episode to feature a Vulcan mind meld.The Devil in the Dark, written by Coon and directed by Pevney, has a fairly simple premise, in which the Enterprise investigates deaths at a mining facility. The investigations reveal a Horta, a hideous looking monster lurking in the caves. But instead of having Kirk and his men battle and defeat the Horta, The Devil in the Dark leads with empathy, having Spock mind meld with the Horta and discover its true purpose, that it merely reacts to the miners who attack it. The Devil in the Dark reminds viewers that understanding, and not conquest, is the true heart of discovery.2. Balance of Terror (Season 1, Episode 14)Before becoming franchise regular Sarek, Mark Lenard played a regal Romulan commander in Balance of Terror. Of course, one actor playing multiple characters will become a standard in Star Trek, even for people who arent Jeffrey Combs. But the connection here is even more powerful, given the big reveal of Balance of Terror, that the mysterious enemy Romulans look just like Vulcans.Written by Paul Schneider and directed by Vincent McEveety, Balance of Terror gives both viewers and Starfleet their first look at Romulans, as the Enterprise and a Romulan ship face off in the Neutral Zone. Against the pressures in their respective organizations, and within the racists on their ship, Kirk and the Romulan captain seek the most peaceful resolution, fully aware that a mistake could result in all-out war. Balance of Terror not only shows a refreshing angle on Kirks tactical genius, but also demonstrates that Treks commitment to empathy extends even to the enemy.1. The City on the Edge of Forever (Season 1, Episode 28)While Kirk was never the womanizer that hes reputed to be, one lady did manage to distract him from his true love, the USS Enterprise. That woman was Edith Keeler, the compassionate and crusading social worker played by Joan Collins. But as Spock tells Kirk in The City on the Edge of Forever, to save the future, Edith Keeler must die.On one hand, The City on the Edge of Forever plays like a standard time travel story, in which Kirk and Spock go back to 1930s New York City in pursuit of McCoy, who goes through a portal after being driven mad in an accident. But the central challenge posed by writer Harlan Ellison (with significant rewrites by an uncredited Coon and Fontana, among others) and director Pevney gets at a key Trek concept: the needs of the many outweighing the needs of the few. As Trek will do time and again, The City on the Edge of Forever shows the human weight of that ethos, forcing Kirk to choose the future over his heart, leading to one of the most striking climaxes in television history.