The Doctor, Tegan and Turlough arrive at an underwater Sea Base on Earth, where a scientific and military team led by Commander Vorshak are monitoring a rival power bloc. The team undergo regular missile launch test sequences to ensure that they are ready at all times to combat an attack. Three Silurians led by Icthar - the surviving member of a Silurian triad - revive a colony of Sea Devil Warriors in order to invade the base and use its weapons to attack the opposing power bloc, thus provoking a global war that will allow the reptiles to conquer the Earth...