Earth, 2084. Two global superpowers hover on the brink of war. When the TARDIS is forced to make an unplanned visit to Sea Base Four, the Doctor, Tegan and Turlough find themselves accused of being enemy agents. Quickly embroiled in a deadly game of paranoid intrigue, it becomes clear that others on the base have sabotage and murder in mind. However, there is a greater threat to humankind: the Silurians and Sea Devils, prehistoric reptiles seeking to reclaim the Earth. Can the Doctor prevent them from implementing their "final solution" and triggering a war that could wipe out the entire human race?