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 centered 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. His efforts are frustrated by a depth charge attack ordered by a pompous politician, Walker, but in the confusion he manages to free a captured submarine and escape back to the surface.
The Sea Devils then capture the Naval base, and the Master has the Doctor taken back to their control center, where he forces him to help finish the machine. The Doctor sabotages the machine and the two Time Lords escape together just as the base is destroyed in an explosion.
The Doctor and Jo head for the sea fort to investigate further. Once there, their boat is destroyed, trapping them inside. They hear the shuffling gait and rasping breath of something approaching them down a corridor.
The Doctor returns to the prison to see the Master and walks into his trap. The two Time Lords fight with swords. The Doctor seems to gain the upper hand but, when he turns his back, the Master pulls a knife and throws it straight at him.
With Jo’s help, the Doctor escapes from the Master’s prison. The two head for the beach, where they find themselves caught between a squad of castle guards, a minefield and a Sea Devil that the Master has summoned from the sea.
The Doctor determines to descend in a diving bell to try to make contact with the reptiles. Jo watches anxiously as the bell is lowered. After a short time contact is lost and it is hauled back up again. Jo looks inside and sees to her horror that the Doctor has gone.
The Master leads an attack by the Sea Devils on the naval base. The Doctor and Jo, on their way with Captain Hart to try to make contact with the reptiles once more, find themselves facing the creatures on the base.
With the Sea Devil base destroyed, the Master contrives to escape in a hovercraft.