In 1930 New York, a delirious McCoy saved social worker Edith Keeler and changed history, until Kirk and Spock allowed her to die and restored the timeline.
In 2267 Leonard McCoy accidentally injected himself with a heavy dose of cordrazine while the Enterprise orbited the planet of the Guardian of Forever. Driven into a paranoid frenzy, he leapt through the time portal, and the starship and the history that produced it vanished.
Kirk and Spock followed him into the past and arrived in New York during the Great Depression. They found work at the Twenty-first Street mission run by Edith Keeler, a compassionate social worker with visions of humanity's future. Spock learned that in true history Keeler died in a traffic accident, but in the altered history she led a pacifist movement that delayed American entry into the Second World War, allowing Germany to win it.
Kirk, who had fallen in love with her, held McCoy back as she was struck by a truck, and history returned to its proper course.