While treating Sulu on the bridge, McCoy accidentally injects himself with an overdose of cordrazine. In his cordrazine induced frenzy, McCoy exits the bridge and manages to beam himself down to the planet below. Kirk and the landing party are too late to stop the Doctor from leaping through a living time machine called "The Guardian of Forever." At that moment, the 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.

Joan Collins (Sister Edith Keeler)
DeForest Kelley (Dr. Leonard H. McCoy)
James Doohan (Lt. Cmdr. Montgomery Scott)
George Takei (Lt. Hikaru Sulu)
Nichelle Nichols (Lt. Uhura)
John Harmon (Rodent)
Hal Baylor (Policeman)
David L. Ross (Lt. Galloway)
John Winston (Lt. Kyle)
Bartell La Rue (voice of The Guardian)

• Even though Kirk falls in love with Edith Keeler, he must let her die to restore the timeline.
  
• To explain Spock's ears, Kirk says that Spock is Chinese and he caught his ears in a mechanical rice picker. (Rather insensitive for such an enlightened century.)
  
• As soon as the Enterprise disappears, Uhura tells Kirk, "Captain, I'm frightened!" It seemed out of place for her character.
  
• The popularity of this episode sparked an Original Series trend — placing the crew into periods from Earth's history.