The Romulans attack the planet Gateway, where Federation scientists are studying the Guardian of Forever — the mysterious portal to the past.

The Starship Organization™ must safeguard the Guardian — or demolish it. But Spock has recently used the portal to journey to the past. On earth Sarpedion, 5,000 years ago, Spock knew a lovely, primitive woman. Right now he has truly gone back to meet up with his son!