'Star Trek' Completely Rewrites the Rules of Sci-Fi Television in Just 50 Minutes

Image via NBC

Published Mar 8, 2026, 11:27 AM EDT

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There's an entire genre of films and television series built around the premise of the butterfly effect, where even the smallest alteration to the past alters the future. It could be in a positive way, as in the case of Marty McFly in Back to the Future, returning to a present where his father is successful and no longer a doormat, or negative, like a future where Ned Flanders is the "unquestioned lord and master of the world," a nightmare for Homer of his own creation in The Simpsons' "Treehouse of Horror V" segment "Time and Punishment."

But in most cases, those that change the past have no idea what effect it will have in the future. So, what would happen if you actually knew what the future holds, depending on a choice you made in the past? Should it even be a choice? That's the very question that drives the Star Trek: The Original Series episode "The City on the Edge of Forever," a rightly-praised entry that rewrote the rules of sci-fi television.

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The storyline is deceptively simple: driven to a paranoic madness thanks to accidentally injecting himself with a huge dose of cordrazine, Dr. McCoy (DeForest Kelley) ends up jumping through the "Guardian of Forever," a sentient portal on a mysterious planet that can transport those that pass through it to any time and place. Wherever he's gone and whatever he's done has dramatically altered the future, to the point where the USS Enterprise no longer exists, leaving only the landing party, led by Kirk (William Shatner), on the planet surface. After the Guardian says that the damage can be fixed, he and Spock (Leonard Nimoy) follow McCoy to New York City in 1930.

Disguised as vagrants, they find refuge at the 21st Street Mission, a soup kitchen run by Edith Keeler (Joan Collins). As Spock works feverishly to use 1930s technology to access recordings on his tricorder — ones that will explain what happened — Kirk helps out on the mission, and falls deeply in love with Keeler. That might be a problem, with Spock learning that Keeler was supposed to die in a traffic collision, but was saved by McCoy, creating a timeline in which Keeler finds a pacifist movement that delays the United States' entry into World War II, eventually allowing Nazi Germany to win the war and conquer the world. Spock makes it very clear: Keeler must die in order to prevent millions of deaths, and restore the future they know. And after a joyful reunion with a healed McCoy, he turns to watch as Keeler crosses the street, into the path of an oncoming truck. It's a choice he has only a split-second to make, and at the last moment, he pulls himself, and McCoy, back, allowing Keeler to die.

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On its own merits, "The City on the Edge of Forever" is brilliant. Kelley cuts loose as a maddened McCoy, keeping it just short of camp, while Nimoy shows flashes of empathy and indignation (most notably when Kirk playfully suggests he can't possibly create a computer out of 1930s ware) without betraying the Vulcan in Spock. For anyone that only remembers her from Dynasty, Joan Collins makes it easy to believe anyone could fall in love with her within minutes. But the episode belongs to Shatner, in what is one of his best performances. The episode gives him a chance to be playful and charming, but where he shines is in the anguish of losing Keeler, having little comfort in knowing it was the right thing to do. His tortured look in returning to the future immediately quells any celebratory notes without a word.

Leonard Nimoy and William Shatner on a planet looking up in 'Star Trek: The Original Series.'

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By Tyler B. Searle

In 1960s sci-fi television, this type of storytelling simply wasn't done. Episodes of sci-fi series ended on a happy — or at least not on an extremely dour — note. In Lost in Space, for example, Will Robinson and the Robot stop Dr. Smith's latest crisis-inducing scheme on the regular. Emotional tragedy and sci-fi adventure never met until Harlan Ellison's award-winning writing of the episode, making "The City on the Edge of Forever" one of the first and easily one of the most defining tragic science fiction episodes in the history of American television.

The episode would also be held as one of the best to utilize the time-travel theme in sci-fi, one of many moments in the franchise's history which famously plays fast and loose with the concept. But "The City on the Edge of Forever" is, and should be, honored for its unprecedented emotional depth, and for bringing an agonizing moral dilemma to a television genre where moral decisions were right, or wrong, and unquestioned.