With the help of the Vulcan leader T'Pau, Spock gets permission to use the Guardian of Forever, a portal through time constructed by a long-vanished race, to venture into the past. On another trip into history, on the planet Sarpeidon, Spock had loved a woman who could not return with him to the future, and now Spock wishes to see the son she bore him, 5,000 years earlier. But a Romulan attack on the Guardian's planet could interfere, unless the EnterpriseTM can keep the Guardian out of their hands.

The Romulans attack the planet Gateway, where Federation scientists are studying the Guardian of Forever -- the mysterious portal to the past. The Starship Enterprise(tm) must protect the Guardian -- or destroy it. But Spock has already used the portal to journey to the past. On the planet Sarpedion, 5,000 years ago, Spock knew a beautiful, primitive woman. Now he has gone back to meet his son!

A cave painting of a Vulcan convinces Spock to journey five thousand years into the past through a time portal on the planet Gateway to reclaim his son.

While the Enterprise is protecting a portal to the past, Spock uses it to travel 5,000 years in the past to meet his son.

The crew of the Starship Enterprise must decide whether to protect or destroy the Guardian of Forever after it is attacked by the Romulans, and the choice is made more difficult since closing the mysterious portal would strand Spock in the distant past.

On the planet Sarpedion, 5000 years ago, Spock knew a beautiful, primitive woman. When the Enterprise is called upon to protect the Guardian of Forever - the mysterious portal to the past - Spock uses it to journey to the past. And to discover his own son.