With Earth locked in a nuclear stalemate, Dr. Who (Peter Davison) again faces the Silurians and the Sea Devils when the creatures attack an underwater military complex, intent on sparking a devastating war between nations in this four-part story. Meanwhile, enemy spies have infiltrated the base and pose a further threat. Can Dr. Who help the Navy battle the aquatic beasties and unravel a web of intrigue among the ranks of the military?