(Played by Joan Collins)
In 1930, during the first part of Earth's 20th Century, Edith Keeler was a social worker who died in a street traffic accident. Centuries later, during a survey using the Guardian of Forever time vortex by the U.S.S. Enterprise, Chief Medical Officer Dr. Leonard McCoy accidentally overdosed on cordrazine. The resulting paranoid psychotic reaction led him to beam down and use the Guardian to escape into Earth's past—again, in 1930s New York City. While there, he saved Keeler from dying, which sparked a chain of events throughout the timeline and essentially changed known 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 Kirk and Spock to follow McCoy into the past to find him "before" he could save Keeler.
Kirk and Spock were indeed able to stop McCoy and thus restored the timeline events to those of their known Prime universe, where the American entry into WW II occured "on time." These events were particularly trying for Kirk, as he had fallen in love with the beautiful and visionary Keeler.