The TARDIS materialises inside Sea Base Four, which lies on one of Earth's ocean beds in the year 2184. The Doctor, Tegan and Turlough quickly run into trouble, as the base's personnel suspect them of belonging to an enemy power bloc. Nearby, a group of Silurian reptiles—who inhabited Earth centuries before mankind—are reviving some of their Sea Devil cousins after an overlong hibernation. The intention of these combined forces is to attack Sea Base Four and provoke a war between rival human governments. The Doctor remembers the carnage that ensued when the intelligent reptiles encountered humans previously, and he is determined to prevent it happening again. But the Silurians are also resolved that, this time, they will not be cheated out of their rightful place as rulers of the planet.