Series: Star Trek: The Original SeriesIn the penultimate episode of the first season (based on a treatment by Harlan Ellison), the crew of the Enterprise finds itself exploring a "temporal disturbance" near an unknown planet. Electromagnetic interference causes the ship's computer to explode, injuring Sulu. McCoy runs to prepare an injection of super-adrenaline for him, but turbulence causes him to accidentally inject himself and become Brainwashed and Crazy. McCoy escapes the bridge and beams down to the planet.Following him, the landing party discovers a city of ancient ruins that appears to be uninhabited and empty except for a mysterious glowing green archway, which McCoy runs through and disappears. The crew notices the archway, which appears to be sentient and exhibits strange telepathic abilities, acts as a viewscreen displaying various scenes from Earth history. The archway, which calls itself the Guardian of Forever, explains that it is a portal to any place and period in history.Upon losing contact with the Enterprise, the crew realize that McCoy has somehow altered the course of history since entering the portal. In order to return to their ship and restore the original timeline, they must follow him and prevent him from doing whatever he did. Steeling themselves to find the correct moment in history, Kirk and Spock jump through the portal and arrive in New York City during The Great Depression, where the pair must disguise themselves while they search for McCoy. Unfortunately, their Starfleet uniforms (and Spock's Vulcan ears) attract unwanted attention. When the pair attempt to steal clothes, they are chased by the police and end up hiding in the basement of a homeless shelter run by the kind, idealistic and smart social worker Edith Keeler (Joan Collins). Keeler offers to take in Kirk and Spock and find them work, although she is slightly suspicious of their military demeanour.While Kirk and Spock do menial jobs, Spock is able to construct a rudimentary computer using electronic parts he has assembled. Kirk and Edith begin to fall in love. While Spock is scanning new stories from their time period, he discovers a terrible truth: Soon after he entered the portal and arrived on Earth, McCoy saved Keeler from dying in an auto accident. Since she did not die as intended, she later became a peace activist and successfully convinced Franklin D. Roosevelt to keep America from entering World War II until too late, allowing the Nazis to win and preventing Starfleet from ever existing. Kirk and Spock realize that in order to prevent this from happening, Edith Keeler must die.Meanwhile, McCoy has been running loose in the city, still affected by the drugs. After an altercation with a homeless man who steals his phaser and shoots himself with it, he is found and taken in by Keeler, who believes he is merely drunk. As he recovers from the drug's effect, he befriends her, unbeknownst to Kirk and Spock.The following night, Kirk takes Edith on a date to the movies — the night of her accident. An offhand comment by Edith reveals to Kirk that she knows McCoy. When Kirk and Spock run into McCoy on the street, Edith walks into traffic to find out what's going on. Kirk instinctively moves to push her out of the way, but Spock insists he not interfere. When McCoy moves to save her, Kirk restrains him, and Edith is struck by a car and dies, returning history to its original timeline.McCoy angrily demands to know why Kirk stopped him from saving Edith. Spock assures him that Kirk knew fully what he was doing. With the timeline restored, the three are able to return to the planet through the portal, where the Enterprise has reappeared. Scotty expresses surprise at the short time that's elapsed since they left. The Guardian offers to show the crew more fantastic journeys, but an emotionally shaken Kirk simply states "Let's get the hell out of here" and prepares to have the crew beamed back aboard.
Episode: Season 1, Episode 28
Title: The City On the Edge of Forever
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Next: Operation: Annihilate!
"Many such journeys are possible. Let me be your gateway."