The City on the Edge of Forever characters Leonard McCoy
Plot
While the USS Enterprise orbits a mysterious planet causing time distortions, Chief Medical Officer Leonard McCoy is treating an injured Lt. Sulu when the Enterprise is rocked by another time wave and McCoy accidentally injects himself with an overdose of cordrazine, a dangerous drug. Delusional and paranoid, McCoy flees from the bridge and beams himself down to the planet. Captain James T. Kirk follows with a search party that includes first officer Mr. Spock, chief engineer Mr. Scott, and communications officer Lt. Uhura. On the planet's surface, they come across an ancient glowing archway which turns out to be a sentient object called the "Guardian of Forever", a portal capable of sending them to any time and place. The Guardian offers to send them back to Earth's past and plays a series of images of historical times which Spock records on his tricorder for research purposes. While the others are distracted, the still-delusional Dr. McCoy bursts out from his hiding place and jumps through the portal. The landing party instantly loses contact with the Enterprise, and the Guardian explains that McCoy has altered the past to such an extent that the Enterprise and the reality they knew no longer exists.
The Guardian says that it's possible to fix the damage, so Kirk requests that it replay the historical period it was showing when McCoy passed through. He and Spock attempt to jump through at a point in time just before McCoy's arrival and find themselves in New York City during the Great Depression. They steal clothes from a fire escape to blend in, and while fleeing from a policeman, hide in the basement of the 21st Street Mission. There they meet the soup kitchen's operator, Edith Keeler. She senses something odd about the intruders but nevertheless offers to pay them to clean up the basement and finds them a place to stay. Later, Spock attempts to discover how McCoy changed history by accessing the recordings on his tricorder. This is a difficult task using the available technology of 1930, which Spock likens to "stone knives and bear skins". While Spock works on the engineering problem, Kirk pays their expenses by doing odd jobs at the 21st Street Mission, where he falls in love with Edith Keeler.
Unknown to Kirk and Spock, McCoy finally arrives in a highly agitated state and stumbles into the mission, where Keeler nurses him back to health. Spock completes his work and discovers that Keeler was supposed to die in a traffic accident but was somehow saved by the arrival of McCoy, creating an altered timeline in which she 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 the first atomic bomb and use it to conquer the world. Kirk is shaken by this revelation and admits his love for Keeler, to which Spock responds that she must die to prevent millions of deaths and restore the future.
While walking to a movie with Kirk the next evening, Keeler casually mentions McCoy. Shocked, Kirk tells her to stay right there while he runs to tell Spock the news. McCoy happens to emerge from the mission at the same time, and the trio reunite on the sidewalk. Observing this, Keeler begins to cross the street to join them and does not notice an oncoming truck. Kirk instinctively turns to save her, but a shout from Spock freezes him in his tracks, and Kirk grabs McCoy so he can't save her, either. Keeler is struck and killed, and a stunned McCoy asks Kirk if he knows what he just did. "He knows, doctor," replies Spock, "He knows."
With history restored, Kirk, Spock, and McCoy return through the Guardian and rejoin the landing party. Mr. Scott is taken aback, explaining that for him and the rest of the landing party, Spock and Kirk had only left "a moment ago" even though the two spent several weeks in the past. A grim-looking Kirk ignores questions about what happened, and when the Guardian declares that "many such journeys are possible", he simply states, "Let's get the hell out of here," and the landing party beams up to the Enterprise.
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