In the more than 50 years since Star Trek made its debut on NBC, the franchise has seen more than a dozen feature films and successfully launched its sixth spinoff series last year with CBS All Access’ Star Trek: Discovery. For all the many amazing stories told in the decades to come, it’s hard to beat the original Star Trek. Running for just three seasons, the series has become synonymous with the science fiction genre and emerged a cultural touchstone that has entertained, educated and inspired dreamers all over the world.

Like any series, Star Trek has its ups and downs. The best episodes, though, rank among television’s very finest. We’ve assembled a list of Star Trek’s 20 best original series episodes, each of which has withstood the test of time in delivering stories that, despite their spectacle and imagination, are ultimately about exploring the human condition. As such, many of the themes explored on Gene Roddenberry’s show have only become more relevant and the show’s 23rd century setting all the more important a future to which we might aspire.

20) The Menagerie - Parts One and Two

star-trek-the-menagerie
Image via CBS

The only two-part episode of the original Star Trek, “The Menagerie” is, in a weird way, a kind of clip show. Before William Shatner was cast as Captain James T. Kirk, Star Trek had shot a pilot, “The Cage,” starring Jeffrey Hunter as Captain Christopher Pike. In fact, the only character to carry over from the unaired pilot was Leonard Nimoy’s Spock. Naturally, he’s front and center of “The Menagerie,” a Starfleet courtroom drama in which Enterprise logs are used as evidence, allowing the entirety of “The Cage” to be incorporated into a larger story set some years later.

Hunter did not reprise the Pike role, the episode finding the character having just suffered an accident that has left him a scarred shell of his former self. With a mysterious motivation that is revealed through the course of his trial, Spock abducts Pike and commandeers the Enterprise. With a course locked to a forbidden planet, Spock calmly turns himself over for his court martial, giving the narrative a fantastic ticking clock.

“The Menagerie” arrived midway through Star Trek’s first season and its expansion of Star Trek lore is, in part, why the franchise continues to this day. Bringing “The Cage” (released some years later on its own) into Star Trek continuity paved the way for future Captains of the Enterprise and reminded us that Star Trek’s timeline doesn’t necessarily need to proceed linearly.

Anyone familiar with Star Trek from the J.J. Abrams films also got to meet Captain Pike. Bruce Greenwood played the part in both 2009’s Star Trek and 2013’s Star Trek Into Darkness.

19) The Corbomite Maneuver

star-trek-corbomite-maneuver
Image via CBS

Although it aired out of sequence, “The Corbomite Maneuever” followed Star Trek’s first two pilots with a story that sees the Enterprise coming into contact with a mysterious and powerful alien sphere in an unexplored area of space. Diplomacy soon fails and a strange looking creature, Balok, tells Kirk that his ship will be destroyed. That’s when Kirk comes up with an epic bluff, aiming to convince the alien that the Enterprise contains a made up element, corbomite, that promises mutually assured destruction.

The notion of bluffing is huge in Star Trek and “The Corbomite Manuever” is the most classic example. A bluff, after all, means applying fiction to create a better reality. In a nutshell, that’s exactly what Star Trek is all about. Over the course of the series, the crew of the Enterprise will use their unique perspective to defy the laws of alien civilizations, Starfleet’s own bureaucracy and even of physics themselves. It’s usually about knowing the right time to do the wrong thing and, of that, Captain Kirk is a proven master. He knows exactly when to bend the rules to achieve the greater good.

“The Corbomite Maneuver” also introduces a common theme that alien races aren’t necessarily as alien as they might appear with a final act that features a young Clint Howard.

18) A Piece of the Action

star-trek-piece-of-the-action
Image via CBS

It may sound a bit silly, but the Enterprise’s visit to a planet ruled by 1920’s Chicago gangsters is a whole lot of fun. It even makes a bit more sense than it sounds: a hundred years before this episode takes place, another ship, the Horizon, wound up bringing a book about Chicago mobsters to the planet’s highly suggestible inhabitants, the Iotians. Treating it like a bible, the entire culture adapted to suit the book.

There’s a lot of costume play throughout Star Trek history and seeing Kirk and Spock in gangster outfits is a whole lot more fun than say, the time they have to dress up the Nazis in the second season episode “Patterns of Force”. “A Piece of the Action” would serve as a prototype for Star Trek: The Next Generation’s holodeck episodes and open the idea that any planet anywhere in the universe could theoretically have a culture identical to any period on Earth.

An abandoned plotline for a 30th anniversary episode of Star Trek: Deep Space Nine (later told in alternate form in a Star Trek comic) would have seen a return to the Iotian planet and the reveal that, following the events of “A Piece of the Action,” the planet wound up being so heavily influenced by the Enterprise’s visit that it had evolved to become, essentially, a planet of classic Star Trek fans.

17) The Squire of Gothos

star-trek-squire-of-gothos
Image via CBS

Several episodes of the original Star Trek find the crew coming face to face with seemingly omnipotent foes, but few are as quite as memorable as William Campbell’s performance as the ebullient mischief maker Trelane, self-professed “Squire of Gothos”. When the Enterprise discovers a planet in an area of space that should be abandoned, the crew is drawn to the world’s sole inhabitant, a godlike individual fascinated with 18th century Earth history who views the crew as nothing more than his playthings.

From Loki of Greek mythology to DC Comics’ Mr. Mxyzptlk, the trickster god is a classic foe. Part of what makes it work so well in the world of Star Trek is because the crew of the Enterprise, to us, appears so advanced technologically. As was the case with foes like Thor or Superman, having an even more powerful foes forces the heroes to rely on their wits alone, ultimately proving that omnipotence is nothing without intelligence and compassion.

Trelane also helped pave the way for John DeLancie’s Q on Star Trek: The Next Generation. Appearing in more than a dozen episodes of subsequent Star Trek series, Q shares Trelane’s paradoxical blend affability and obnoxiousness. While it’s never specified in the canon, stories told in Star Trek books and comics have connected the characters.

16) Day of the Dove

star-trek-day-of-the-dove
Image via CBS

The Enterprise responds to a distress call only to find a deserted planet and Klingon forces nearby. When both ships somehow become disabled, tensions begin to mount to an unusual degree. Walter Koenig’s Chekov is threatening to avenge the death of his brother at the Klingons’ hands. But then it’s revealed that he doesn’t even have a brother. Things get weirder and weirder as the planet itself seems to encourage conflict, supplying weapons and easy reasons to give into hatred.

It’s never revealed exactly what the force is that both crews encounter on planet Beta XII-A, but it seems to be a destructive energy that is, perhaps, a manifestation of destructive energy itself, depicted as a crackling red force. When Kirk realizes that he and Michael Ansara’s Klingon commander Kang are being manipulated, “Day of the Dove” gives us one of William Shatner’s great mini-monologues.

“All right. All right,” Kirk shouts at Kang. “In the heart. In the head. I won't stay dead. Next time I'll do the same to you. I'll kill you. And it goes on, the good old game of war, pawn against pawn! Stopping the bad guys. While somewhere, something sits back and laughs and starts it all over again.”

That’s a message that, sadly, is every bit as timely today as it was half a century ago.

15) The Galileo Seven

star-trek-the-galileo-seven
Image via CBS

Things go bad during a routine science mission, forcing a shuttlecraft, the Galileo, to make an emergency landing on a dangerous planet, home to enormous apelike beasts. What’s more, a coming ion storm and trouble in another part of the galaxy mean that the Enterprise may need to give up the search.

While there’s some great Kirk moments as he squeezes every possible opportunity from the chain of command to keep looking for the Galileo, this episode is Spock’s show. We get to see him take command over a six-person crew and deal with having to give orders that put officers in mortal danger. Fear amongst the crew makes things all the more dangerous as Spock has to face officers who question his Vulcan logic and, ultimately, his own uncertainty in command to save the crew.

The plot for “The Galileo Seven” originated with Oliver Crawford, who co-wrote the episode’s script with S. Bar-David. He has said that “The Galileo Seven” was directly inspired by the 1939 big screen thriller Five Came Back, about a small airplane that crashed in a South American jungle.

14) A Private Little War

star-trek-private-little-war
Image via CBS

Star Trek was offering a direct allegorical take on the Vietnam War in 1968 with a story that finds Captain Kirk in a moral dilemma. A planet of immense natural resources, Neural, is home to a primitive race. Although Starfleet’s Prime Directive would normally preclude any interference, the Enterprise learns that conflict has broken out among the natives with one side being given advanced weaponry by the Klingon Empire.

While the Enterprise quite often finds itself in the position of having to balance moral imperatives, “A Private Little War” is the best example of the show taking on a contemporary political issue, even if the conclusion doesn’t offer any easy answers.

“A Private Little War” also introduces a memorable Star Trek alien that only appears in this one episode. The Mugato is a ferocious white ape with a spiked back and a poisonous bite. Ben Stiller, a big Star Trek fan, borrowed the name for Will Ferrell’s character in his 2001 comedy Zoolander.

13) Journey to Babel

star-trek-journey-to-babel
Image via CBS

The Enterprise is journeying to the planet Babel for a diplomatic conference when one of the visiting ambassadors is murdered. The lineup of suspects includes a wide range of alien dignitaries and one chief suspect: Mark Lenard’s Vulcan ambassador Sarek. Adding to the drama, Sarek just happens to be Spock’s estranged father.

Not only is “Journey to Babel” an interesting mystery with a grand assembly of interesting alien species, it reveals so much about Spock and his history with his father. It’s a relationship that continues to develop for decades after “Journey to Babel”. Lenard returns as Sarek throughout the franchise’s big screen run and beyond. He’s even set to be a featured character in Star Trek: Discovery with James Frain filling in for the late Lenard.

Before he played Sarek, Mark Lenard famously took on the role of another prominent Star Trek character. Look for details on that performance a bit further down this list.

12) Let That Be Your Last Battlefield

star-trek-let-that-be-your-last-battlefield
Image via CBS

The Enterprise intercepts a stolen shuttlecraft containing Lou Antonio’s Lokai, a fugitive from a planet called Charon. Although he looks like a human being, Lokai is split down the middle, one side black and the other white. It’s not long before Frank Gorshin shows up as Bele, a fellow being from Charon who utterly despises Lokai. Although they may initially appear identical, it is revealed that Bele and Lokai are alternately colored. Bele is black on the left side and white on the right while Lokai is the reverse. Hence, in their culture, their hatred for one another.

With “Let That Be Your Last Battlefield,” Star Trek finds a way to tell a story that isn’t just about racism, but that makes racism itself the focal point for its sci-fi lens, imagining a brand new way to hate and making a powerful reminder that any reason for doing so based on any other physical attribute is just as unnatural.

11) The Doomsday Machine

star-trek-the-doomsday-machine
Image via CBS

The crew of the Enterprise faces one of its most awesome cosmic challenges when it comes up against a massive, world-eating device of extreme alien origin. In fact, the device is so alien that we never really learn what it is, although Kirk theorizes it to be an ancient doomsday device. The Enterprise isn’t the first Starfleet ship on the scene this time, either. By the time the Enterprise discovers the danger to the galaxy, the USS Constellation has already risked everything in an attempt to stop the planet killer. The Constellation is recovered by the Enterprise with only one crewmember still left alive, William Windom’s Commodore Matt Decker.

Not only does “The Doomsday Machine” feature such a memorable monster, it features some great drama between Kirk and Decker. Having just lost his crew to the cosmic goliath, the Commodore is suffering from severe posttraumatic stress and not necessarily thinking with a clear head. Because he technically outranks Kirk, that poses a serious problem in dealing with the matter at hand.

“The Doomsday Machine” also sets up a nice bit of continuity with Star Trek: The Motion Picture. One of the main characters in the 1979 Robert Wise film is Stephen Collins’ Captain Willard Decker, the son of Commodore Matt Decker.

10) The Devil in the Dark

star-trek-devil-in-the-dark
Image via CBS

Something is killing workers on distant a mining colony visited by the Enterprise. Deep in the caves of Janus VI, a rocklike creature is melting people alive with its molten abilities. Despite the attacks, though, there’s no trace of the creature and no understanding for how a carbon based life form could exist in such an environment.

“The Devil in the Dark” functions as both a sci-fi slasher and a moral tale. The end of the episode reveals that life sometimes manifests itself in forms that we may not have ever considered, brought to life visually when Spock performs a Vulcan mind-meld with the alien creature, a horta, and learns that its motivations are not malicious and that, instead, it is the miners who have unwittingly done a great wrong to it.

Although the horta marks another iconic Star Trek alien, “The Devil in the Dark” is their sole appearance of the species the franchise outside of very minor nods in later spinoffs.

9) By Any Other Name

star-trek-by-any-other-name
Image via CBS

The Enterprise encounters a pair of scouts from the Kelvan Empire, mighty beings from the Andromeda Galaxy who want to use the Starfleet vessel on a generational voyage to their homeworld. Armed with powerful belts that can, among other things, transform people into easily destroyed little polyhedrons of their basic genetic makeup, they easily take the ship. Only a few officers are left in their human form and it’s humanity that, as it often does, gives the Enterprise crew their edge. Because the Kelvans have taken human form for their journey, they’re not used to some of the finer points of being human.

“By Any Other Name” has James Doohan’s Scotty teaching the male Kelvan about alcohol while Kirk teaches the female one about love. There’s a major degree to which Star Trek celebrates the clash of human and alien cultures and here we get to see the whole crew doing so to the best of their specific abilities.

The work of William Shakespeare is something referenced quite a bit throughout the Star Trek franchise. This episode references a line in Romeo and Juliet wherein Juliet argues that a rose is a rose because of the form it takes and not what it’s called.  Star Trek expands that idea with the suggestion that anything in the form of mankind will, in doing so, become mankind.

8) A Taste of Armageddon

star-trek-a-taste-of-armageddon
Image via CBS

“A Taste of Armageddon” offers a brilliantly high concept sci-fi plot: the Enterprise visits a world that has evolved beyond destructive combat, but not beyond war itself. Instead of destroying one another with weapons that would threaten to also destroy their culture, the conflicting sides of Eminiar VII’s population have agreed to wage war through a highly accurate simulation. If a resident is in an area that the simulation has deemed destroyed, he or she is expected to immediately turn themselves in for disintegration. Unfortunately, a few Enterprise crewmembers are unwittingly present when a building is marked destroyed and their refusal to surrender their lives could mean that a more destructive form of war returns to the planet.

Most of the time, the crew of the Enterprise comes across planets whose values aren’t quite up to date with those held by the enlightened Starfleet. Here, though, the crew is forced to deal with a planet that actually makes a pretty good point and achieved what seems to be far less destructive form of combat. Kirk makes an interesting argument, however, suggesting that the people of Eminiar VII have, in attempting to mute the destructive effects of war, helped mute the horror of it as well. By the time the credits roll on “A Taste of Armageddon,” you won’t be wrong if you’re not exactly thinking of Starfleet as the good guys this time.

7) Space Seed

star-trek-space-seed
Image via CBS

“Space Seed” introduced Star Trek’s most famous antagonist, Ricardo Montalban’s genetically enhanced 20th century superman Khan Noonien Singh. The Enterprise comes across a long-lost vessel, the Botany Bay, that contains Khan and 84 of his crew. Soon, Khan is using his enhanced cunning to take control of the Enterprise.

One of the reasons Khan works so well as a foil for Kirk is because he’s both physically and mentally superior to the Starfleet Captain, but also shares Kirk’s charisma. He’s an easy bad guy to root for and his backstory raises some interesting questions about what Earth’s Eugenics Wars were like in the 1990s.

While “Space Seed” is Khan’s sole appearance in the series, Montalban would famously reprise the role for Nicholas Meyer’s Star Trek II: The Wrath of Khan in 1982. Benedict Cumberbatch would then play an alternate reality version of the character in Star Trek Into Darkness.

6) Arena

star-trek-arena
Image via CBS

The Enterprise comes into territorial conflict with the Gorn, a race of reptilian creatures with whom Starfleet has never dealt. When conflict threatens to breaks out with the Gorn ship, both vessels are disabled an incredibly powerful race called the Metrons. To settle the issue, the Metrons will have Captain Kirk face off against the Gorn captain on a rocky desert planet. Even though the Gorn easily outmatches the Starfleet Captain physically, Kirk has a few tricks up his sleeve.

As with “A Taste of Armageddon,” “Arena” focuses on a transference of a larger conflict into a smaller one. While the former dealt a bit more with the ethics in doing so, “Arena” places more like a sporting event. On the bridge of the Enterprise, the crew is helpless to do anything but watch as Kirk and the Gorn captain battle to the death.

“Arena” has become one Star Trek’s most famous episodes primarily due to its incredible Gorn costume. While it has a degree of 60s sci-fi cheese, it still looks quite a bit better than attempts to later bring back the Gorn species with CGI on Star Trek: Enterprise.

5) The Trouble with Tribbles

star-trek-trouble-with-tribbles
Image via CBS

One of Star Trek’s most famous episodes brings the Enterprise to Space Station K-7 where there is some fear that visiting Klingons might disrupt the delivery of much a needed grain to an Earth colony. Meanwhile, Nichelle Nichols’ Uhura goes shopping and purchases a Tribble, a small purring creature that looks like a ball of hair. Unfortunately, it turns out that Tribbles are born pregnant and they begin to multiply exponentially.

“The Trouble with Tribbles” is a fairly light-hearted adventure that also feels like a day in the life for the crew of the Enterprise. Kirk’s attempts to deal with Federation politics wind up taking a back seat to the tribble problem as a memorable Star Trek species is born.

For the franchise’s 30th anniversary, Star Trek: Deep Space Nine would return to “The Trouble with Tribbles” with an episode called “Trials and Tribble-ations”. In it, the crew travels back in time to prevent a plot to assassinate Captain Kirk during the events of “The Trouble with Tribbles,” digitally inserting actors into the background of the original episode with an adventure that makes use of impressive visual effects to tell a concurrent narrative.

4) Mirror, Mirror

star-trek-mirror-mirror
Image via CBS

Quite arguably the definitive pop culture example of parallel realities, “Mirror, Mirror” finds Kirk, McCoy, Scotty and Uhura in a transporter accident that swaps them with their counterparts from an alternate timeline. While the Enterprise is negotiating for dilithium on behalf of the United Federation of Planets in the regular timeline, the alternate history sees an aggressive Enterprise take the valuable resources by force in the name of the Terran Empire.

“Mirror, Mirror” is particularly famous for having a goateed Spock in the mirror universe. The “evil” goatee has become iconic in pop culture to represent all kinds of evil twins and alternate reality doppelgangers.

Although there were several novels and comic books that made use of the Mirror Universe, it would be nearly three decades before the timeline would return in the official Star Trek canon. The second season Star Trek: Deep Space Nine episode “Crossover” would make visits to the parallel timeline a nearly annual event for the spinoff series.

3) Balance of Terror

star-trek-balance-of-terror
Image via CBS

When the Enterprise encounters a Romulan vessel, the two ships come into combat and soon leave one another disabled and floating in space. Each races to make repairs before the other to claim victory in an encounter that reveals surprising new details about an old Starfleet foe.

In the Star Trek timeline, Earth experienced a brief but intense war with the Romulan Star Empire about a century earlier. Because the combat occurred in space, Earth never learned what Romulans look like. Here, it’s revealed that they’re a warlike offshoot of the Vulcan species, leading to some immediate tension between Spock and one of his crewmates.

A throwback to submarine thrillers like The Enemy Below and Run Silent, Run Deep, “Balance of Terror” is elevated by Mark Lenard’s pre-Sarek appearance as the commander of the Romulan vessel. As the viewpoint shifts from the Enterprise to the Romulan vessel, we find that he and Kirk are not so different in their adherence to their duty.

2) Amok Time

star-trek-amok-time
Image via CBS

The first episode of Star Trek’s second season reveals a Vulcan secret. Every seven years, the species goes through a mating cycle in which they must return to Vulcan and take a mate. Unfortunately, Spock’s would be mate demands the koon-ut-kal-if-fee, a battle to death between her suitor and a champion of her choosing. When she chooses Captain Kirk, both officers must fight in a Vulcan ceremony that will not end until one of them is dead.

Not only does “Amok Time” deliver on its promise of an epic Kirk vs. Spock fight to the death, but it’s a great example of Spock growing to trust and appreciate his crewmates. It’s an intensely personal story for Spock and it culminates with a fantastic line from the character.

“After a time,” Spock tells a Vulcan woman who has rejected him in favor of another suitor, “You may find that having is not so pleasing a thing after all as wanting. It is not logical, but it is often true.”

For that line alone, “Amok Time” deserves its honored place in Star Trek history.

1) The City on the Edge of Forever

star-trek-city-on-the-edge-of-forever
Image via CBS

It’s pretty much universally understood that the penultimate episode of Star Trek’s first season is the franchise’s very finest story. Having accidentally injected himself with a drug that drives him crazy, DeForest Kelley’s Dr. McCoy beams down to an alien planet where an ancient gateway, the Guardian of Forever, allows passage through time and space itself. A crazed McCoy travels back to Earth in the 1930s and does something that stops Starfleet from having ever existed. With no other choice, Kirk and Spock travel back themselves in the hopes of restoring the timeline.

It’s in the 1930s that Kirk and Spock meet Joan Collins’ Edith Keeler a thoroughly lovely young woman who has dedicated herself to helping the less fortunate in a New York Mission. As they search for McCoy, Kirk begins to develop a relationship with Edith. And then the bad news hits: in order to fix the timeline, Edith Keeler must die.

The need for Edith Keeler to die is made all the more tragic by the fact that she’s not in any way a bad person. Spock explains that her success at organizing a pacifist movement in the United States will lead to the United States delaying its entrance into World War II. It is peace that she’s fighting for and in the name of peace that she must be sacrificed.

“The City on the Edge of Forever” is, simply, the epitome of what science fiction has to offer, bolstered by an original script by legendary author Harlan Ellison and a perfect romance between William Shatner and Joan Collins.

“Let me help,” smiles Kirk to Keeler as they a New York street together, he comments on her choice of words. “A hundred years or so from now, a famous novelist will write a classic using that theme. He'll recommend those three words, even over 'I love you’.”

For all the franchise’s many adventure across space and time, none come quite close to the simple, tragic elegance of “The City on the Edge of Forever”.