Jon Pertwee's Doctor encounters ancient reptiles from beneath the sea in the original soundtrack of this classic 1972 TV adventure, with linking narration by Katy Manning. 'We shall destroy Mand and reclaim the planet!' Visiting the Master in his high-security prison just off the south coast of England, the Doctor and Jo Grant's curiosity is piqued by a recent spate of disappearing ships.

Investigating a nearby sea fort, they encounter a ferocious amphibian creature - the cousin of a reptile race whom the Doctor met once before. These 'Sea Devils' have been revived from hibernation by the Master, whose scheme is being aided by the Prison Governor.

Realising that the Master wants to create havoc on Earth, and to see humanity subjugated by the Sea Devils, the Doctor once again attempts to broker peace between the two races.