Edith Keeler, human. In 1930, during the first part of Earth's 20th Century, Keeler was a social worker who died in a street accident. Centuries later, during a survey of the Guardian of Forever by the U.S.S. Enterprise, the ship's Chief Medical Officer, Dr. Leonard McCoy, during a temporary bout of mental illness brought on by an accidental injection of cordrazine, escaped into Earth's past. While there, he saved Keeler from dying, which sparked a chain of events throughout the timeline, essentially changing history.

In the altered timeline, Keeler, a passionate believer in peace, spearheaded a movement which delayed the entry of the United States into World War II, thus paving the way for an Axis victory. Ultimately, Starfleet, the Federation and the U.S.S. Enterprise were no longer in existence, but the proximity to the Guardian of Forever by the landing party allowed Captain James T. Kirk and First Officer Spock to follow McCoy into the past.

Kirk and Spock were able to prevent McCoy from saving Keeler's life, thus restoring the timeline. These events were particularly trying for Kirk, as he had fallen in love with Keeler.