TOS Season 1 Episode 28 The City on the Edge of Forever

Essential Star Trek. Not only does this episode provide a thoroughly entertaining ride, it subverts expectations in a number of ways unusual to the original series. It’s one of the first layers Trek adds to color it’s idealistic premise. The plot is expertly structured to maximize the arrival of Bones, who has numerous great scenes. Kirk is also put in a genuinely troubling position, having to make a choice the viewer has a visceral connection to.

Plot

While the USS Enterprise is orbiting an unexplored planet, Chief Medical Officer Leonard McCoy is treating an injured Lt. Sulu when the Enterprise is rocked by a time distortion and McCoy accidentally injects himself with an overdose of cordrazine, a dangerous drug. Delusional and paranoid, McCoy flees from the bridge to the transporter room, beaming himself down to the planet below. Captain James T. Kirk (William Shatner) leads a landing party to look for McCoy, and they come across an ancient glowing stone archway, which turns out to be the cause of the time distortions. They discover the archway to be sentient, and this "Guardian of Forever" (voiced by Bartell LaRue) explains that it is a doorway to any time and place. While Spock (Leonard Nimoy) is recording historic images from the portal, McCoy escapes through it. The landing party suddenly loses contact with the Enterprise, and the Guardian informs them that McCoy has altered the past, and that the Enterprise, and all that they knew, no longer exists.
The Guardian permits Kirk and Spock to follow McCoy in an effort to repair the timeline. Spock times their passage so as to arrive where McCoy did ahead of when he will arrive, and they find themselves in New York City in 1930, during the Great Depression. After stealing clothes from a fire escape to blend in, they meet a woman named Edith Keeler (Joan Collins), who runs the 21st Street Mission. They are given a place to sleep, along with doing odd jobs to earn money. Spock works to devise a method of interfacing with his tricorder and analyze its recorded images to determine how McCoy has altered history. While they await his arrival, Kirk and Keeler spend time together, and Kirk begins to fall in love.
McCoy arrives, and stumbles into the mission, unnoticed by Kirk and Spock, and Keeler nurses him back to health. Spock completes his work and discovers Keeler was supposed to die that year in a traffic accident. Spock learns Dr. McCoy altered the past by saving Keeler's life and, in the altered timeline, Keeler founded a pacifist movement on the eve of World War II. This causes the United States to delay its entrance into the war, allowing Nazi Germany time to develop nuclear weapons, which they launch in their V-2 rockets to conquer the world. Kirk admits his love for Keeler, and Spock answers that Keeler must die in order to prevent millions of deaths and restore the future.
Going with Kirk to see a movie, Keeler mentions McCoy. Kirk, shocked and excited, tells her to stay where she is and calls Spock to tell him this. The Starfleet trio reunite in front of the mission. Observing this and curious, Keeler crosses the street to join them, and she steps right in front of a fast-moving truck. Kirk turns to save Keeler from the truck, but a shout from Spock freezes him in his tracks. Then Kirk blocks McCoy from saving her and she is struck and killed. McCoy, not knowing why Kirk and Spock did what they did, is stunned and in disbelief that Kirk had knowingly stopped him from saving Keeler.
With history restored, Kirk, Spock, and McCoy are returned to the Guardian's planet where the rest of the landing party is waiting. Scotty remarks, as Kirk, Spock, and McCoy return through the portal, that the 3 had only left "a moment ago", even though, for Spock, Kirk, and McCoy, much more time had passed than that. When the Guardian declares that "many such journeys are possible", a brokenhearted Kirk simply states "Let's get the hell out of here," and the landing party beams off the planet back to the Enterprise.