A thousand-year feud between sworn enemies threatens to destroy the Enterprise.

The Enterprise discovers a planet that defies the conventional theories of planetology, so Kirk, Sulu, McCoy and D'Amato (a geologist) beam down to investigate. As they depart, a mysterious woman appears in the transporter room and attacks the chief there. Then the Enterprise is hurled nearly a thousand light years away! The strange female apparition reappears, and demonstrates that she is a grave threat to both stranded crew and the ship...