A time portal leaves Kirk, Spock and McCoy stranded in different eras of a dying planet.
The Enterprise visits Sarpeidon, whose sun is about to become a nova, and discovers that its people have escaped by going to the past through a portal known as the Atavachron. Kirk is sent into a world resembling the seventeenth century, while Spock and McCoy are sent to an ice age, where Spock's behaviour changes.
The story has Spock regress to a more emotional, primitive Vulcan, and he falls in love with a woman named Zarabeth. It is a poignant story of time, loss and identity. Fans remember it for its depiction of Vulcan history, the library, and the sacrifice of the doomed.