The 10 standout episodes of Star Trek: The Original Series

"Where No Man Has Gone Before" (Season 1, episode 3)

After the Enterprise goes through an energy rift at the edge of the galaxy, Captain Kirk's (William Shatner) friend and shipmate, Lt. Commander Gary Mitchell (Gary Lockwood), begins to develop terrifying ESP abilities that grow stronger by the minute. As he makes the transition from man to "god," he becomes increasingly more dangerous and detached from humanity. Ship psychiatrist Dr. Elizabeth Dehner (Sally Kellerman) believes his mutation can help mankind evolve, but Spock (Leonard Nimoy) is adamant he must be killed before he destroys them all. Will Kirk choose his best friend over the best interests of his crew — and the universe?

"Where No Man Has Gone Before" is most famous for being the second pilot filmed for the series, and introducing viewers to Captain James T. Kirk, Chief Engineer Scotty (James Doohan), and Lieutenant Sulu (George Takei). It's also just a great hour of sci-fi storytelling. This superior first episode seamlessly mixes action, high stakes emotions, and tough ethical questions, setting the blueprint for the franchise.

"The City on the Edge of Forever" (Season 1, episode 28)

When a time disruption from a nearby planet rocks the Enterprise, Dr. McCoy (DeForest Kelley) — aka Bones — accidentally injects himself with an overdose of a dangerous drug. Driven mad, he flees to the planet below and goes through the time warp, changing history and erasing the Federation of Planets from existence. Kirk and Spock follow him to set things right, and find themselves in 1930s Depression-era New York.

As they search for Bones, Kirk meets and falls in love with a social worker named Edith Wheeler (Joan Collins) whose fate, it turns out, will determine the course of humanity. Once again, Kirk must choose between someone he loves and the greater good. One of Trek's most emotionally charged hours, "The City on the Edge of Forever" — scripted by Harlan Ellison — is considered by many to be the greatest episode of all-time.

"Space Seed" (Season 1, episode 22)

The Enterprise team stumbles upon the marooned S.S. Botany Bay in deep space and awakens the crew from suspended animation. They soon discover these lost spacefarers were exiled from Earth during the infamous Eugenics Wars of the 1990s, and our heroes have unwittingly unleashed a genetically enhanced super-tyrant named Khan Noonien Singh (Ricardo Montalban) in the 23rd century.

Montalban's magnetic, calculating would-be-ruler serves as a perfect foil for Shatner's compassionate, tactical Kirk, leading to a great one-on-one showdown between the two for control of the ship. This season one episode is the introduction of the series' most infamous villain, who 15 years later will headline Star Trek II: The Wrath of Khan, widely regarded as the best film in the franchise.

"Amok Time" (Season 2, episode 1)

Kirk vs. Spock! Kirk's shirt ripped open (again)! Spock in heat! This episode has everything a fan of the duo that launched a thousand slash fics could want. Every seven years, a Vulcan must return home for an ancient mating ceremony called "pon farr." When Kirk and Bones accompany him, they find themselves dealing with (farr) more than they bargained for when the Captain is forced to battle Spock in a ritual fight to the death.

In addition to the showdown between the leads, "Amok Time" has several other firsts: the first use of the Vulcan Salute, the first appearance of Ensign Pavel Chekov (Walter Koenig), and the first glimpse of the planet Vulcan itself. This perennial favorite is also heavily referenced in Star Trek: Strange New Worlds' fifth episode — titled "Spock Amok" — on Paramount +.

"Mirror, Mirror" (Season 2, episode 4)

An away team consisting of Kirk, Bones, Uhura (Nichelle Nichols), and Scotty are sent to an alternate dimension when they are caught in an ion storm mid-transport. This "mirror" dimension is populated by violent doppelgangers of the Enterprise crew who serve the Terran Empire instead of the Federation of Planets. The foursome must navigate the cutthroat nature of this universe and find a way back before their secret is exposed — all while surviving a mutiny to overthrow this reality's Kirk. Hip daggers, bare midriffs, and Spock's goatee are just a few of the signs that things in this universe are askew.

Although Evil Bearded Spock is certainly fun to see, George Takei gives the standout performance as a delightfully evil Sulu, complete with a badass facial scar. This episode is a highlight of the original series and forms the foundation for several stories in future Trek franchises like Deep Space Nine, Enterprise, and Discovery.

"The Trouble with Tribbles" (Season 2, episode 15)

A fan favorite episode, "The Trouble with Tribbles" is a comedic left turn that shouldn't work, but absolutely does. Starring adorable little furballs who are "born pregnant" and multiply at a rapid pace, this zany hour allows the heady sci-fi questions of morality to take a back seat in favor of punchlines and hijinks.

The actual plot of the episode revolves around Kirk protecting a supply of space grain essential to Starfleet's sovereignty over a contested planet. The fun begins, however, when Uhura picks up one of the tiny tribbles while on shore leave at Space Station K-7 and unleashes an infestation on every corner of the Enterprise — including Kirk's lunch. The Captain's exasperated responses to the growing tribble crisis are comedy gold, especially as he seems to be the only one immune to their cooing charms. "Tribbles" also features a fantastic slapstick bar fight between Scotty, Chekov, and a handful of Klingon officers for the honor of the Enterprise. This episode is a fun detour into the lighter side of the crew's five-year mission.

"Balance of Terror" (Season 1, episode 14)

Kirk and company find themselves in an action-packed showdown with the Romulans when they investigate a mysterious loss of communication with Federation outposts near the Neutral Zone. Despite a history of war with Earth, no one has ever laid eyes on an actual member of their species — until now.

As the Romulans are believed to be the violent cousins of the Vulcans, Spock becomes the subject of suspicion and xenophobia from some of the crew — particularly Lt. Stiles (guest star Paul Comi) — whose ancestors were killed in the Earth-Romulan War. The story's point-of-view shifts between the two vessels, allowing the viewer to see that the warring foes are more alike than they realize.

This season one classic features the first appearance of the Romulans — who will go on to be recurring antagonists for the heroes of the franchise. Also, keep an eye out for actor Mark Lenard as the Romulan Commander. He'll return to the series in a recurring role as Spock's father.

"The Corbomite Maneuver" (Season 1, episode 10)

While exploring an uncharted area of space, the Enterprise is pursued by a mysterious cube emitting harmful radiation. To protect themselves, they lay waste to it, and incur the wrath of Balok, commander of a technologically superior alien race. He takes control of the Enterprise's systems and declares the ship, and everyone onboard, will be destroyed in 10 minutes. A tense hour with a really wild twist ending, this installment showcases Kirk's ingenuity and characteristic refusal to lose — one of the many times the Captain will boldy bluff where no man has bluffed before.

"The Corbomite Maneuver" is also the first time DeForest Kelley (McCoy) and Nichelle Nichols (Uhura) played their iconic characters, although they appear earlier in the series due to NBC originally airing episodes out of production order.

"The Doomsday Machine" (Season 2, episode 6)

Our intrepid explorers receive a distress signal from fellow Starfleet ship the U.S.S. Constellation and rush to its aid. Upon arrival, they find Commodore Matthew Decker (William Windom) — the ship's commander and sole survivor — wracked with guilt and suffering from PTSD. Decker's entire crew was annihilated by a massive energy weapon of unknown origin that destroyed the entire star system.

Pulling rank, he takes command of his rescuer's ship, and puts the crew of the Enterprise in the crosshairs of the unstoppable world-killing device. Kirk, marooned on Decker's derelict starship, must figure out a way to rescue the Enterprise from both the machine and an out-of-control superior officer. Introducing a planet-ending energy weapon 10 years before Star Wars, this episode features a real nail-biter of an ending.

Fun fact for the continuity-conscious: Decker's son, Willard Decker (Stephen Collins), will play a prominent role in 1979's Star Trek: The Motion Picture.

"Arena" (Season 1, episode 18)

Captain Kirk, Spock, Bones, and an unfortunate "Redshirt" arrive at the Cestus III Outpost for a diplomatic mission and find it annihilated by an alien race called the Gorn. Seeking revenge, Kirk pushes the Enterprise to pursue the Gorn starship and destroy it. The chase leads into an unmapped sector of space ruled by a powerful force calling themselves the Metrons. Outraged by the brutality of both ships, the Metrons force the two captains to settle their dispute in a fight to the death on a desert planet. The winner will leave the sector unharmed, while the loser, and their crew, will die.

The bulk of this episode involves William Shatner being chased by an actor in a giant rubber lizard suit — and honestly, it rules. Yes, the suit looks goofy, but the showdown is fun as hell and the message of the story is classic Star Trek: sometimes there is more going on beneath the surface than we realize. Very few Star Trek villains are ever just one-dimensional bad guys, and the Gorn are no exception.

Fun fact: Ted Cassidy, who provides the voice for the Gorn captain, also provided the voice of Balok in "The Corbomite Maneuver."