The City on the Edge of Forever (Episode)
Kirk and Spock go back in time to restore history.
Dr. McCoy accidentally injects himself with an overdose of Cordrazine. Delirious, he transports down to the planet below, to which the Enterprise has come to investigate strange ripples in time. Beaming down with a landing party, Kirk and Spock discover a living machine known as the Guardian of Forever. When McCoy leaps through the machine's portal, he vanishes into Earth's past. History is changed and the Enterprise ceases to exist. Hoping to undo the damage to history, Kirk and Spock leap through the device, into Depression-era New York City. There, Kirk meets and falls in love with Edith Keeler, a progressive social worker. Spock manages to use vintage 1930 equipment to construct a tricorder monitoring device, and while replaying footage from the machine, he discovers that Keeler has two possible futures: she will either begin a pacifist movement that will delay America's entry into World War II long enough for the Nazis to win, or she will die in a traffic accident. Kirk commits the most difficult act of his life when he prevents McCoy from saving her. Returning through space and time, Kirk, Spock, and McCoy discover they succeeded, and history has been restored.