The Enterprise is on a peaceful mission at Starbase 12 when a bizarre cosmic phenomenon causes a Kilingon ship to suddenly vanish - with Spock aboard for the ride. Spock's last message from the Klingon ship is cryptic and frightening. The Klingons are travelling into the past, searching for the one man who holds a vital clue to the future. If they can kill the man, the course of history will be changed - and the Federation will be destroyed!