himself with an overdose of cordrazine, a drug which makes him exhibit
signs of paranoia and madness, while treating an ailing Sulu on the
Bridge. Delirious, he beams down to a nearby planet's surface, with
Kirk and a landing party on his heels.
too late to stop the
doctor from leaping through a living time machine called "The Guardian
of Forever." At that moment, the U.S.S. Enterprise ceases to exist and
the landing party is stranded. The Guardian explains that McCoy went
back into Earth's history and changed it, thereby altering the future.
Kirk and Spock go through the Guardian, to Depression-era America, a
few days before McCoy is to arrive and change history.
encounter a social worker,
Edith Keeler, who helps them find work to pay for the equipment Spock
requires to build a tricorder. Unknown to Kirk and Spock, Edith has
taken in the recently-arrived and ill McCoy. Kirk promptly falls in
love with Edith and is devastated when Spock completes his tricorder
and discovers that in order to repair history, they must let Edith
Keeler be killed in an auto accident. If they allow McCoy to save her
as he did before — she will start an effective pacifist
will delay the United States' entrance into World War II, thus allowing
Hitler's Germany to develop the atomic bomb first and conquer the
moment comes, a
heartbroken Kirk stops McCoy from saving Edith, and the three officers
journey back through the Guardian, where they find things as they
should be again.