The Doctor and Jo visit the Master in his high-security prison on an island off the south coast of England and hear from the governor, Colonel Trenchard, that ships have been mysteriously disappearing at sea. Investigating, the Doctor learns from Captain Hart, commander of a nearby Naval base, that the sinkings have centred around an abandoned sea fort. He and Jo then visit the fort and are attacked by what one of the men there terms a Sea Devil - an amphibious breed of the prehistoric creatures encountered by the Doctor shortly after his exile to Earth.
The Master, aided by a misguided Trenchard, is stealing equipment from the Naval base in order to build a machine to revive the Sea Devils from hibernation. The Doctor takes a diving bell down to the Sea Devils' underwater base to try to encourage peace...