After accidentally overdosing on a powerful stimulant, Dr. McCoy becomes unbalanced and disappears through the guardian of forever, a newly discovered time portal on a remote planet. Kirk and Spock follow after learning that McCoy somehow changed history, removing everything they once knew; including the Enterprise. Arriving in the 1930s, the duo meet Edith Keeler, a New York social worker who gives them a place to stay. As the days pass, and McCoy is nowhere to be seen, Kirk finds himself falling in love with Keeler… but Spock discovers that Keeler must die to restore the timeline