The Doctor and Jo visit the Master in his high-security prison on an island off the south coast of England. The governor, Colonel Trenchard, says ships have been disappearing mysteriously at sea. The Doctor discovers that Trenchard and the Master are in league to contact the Sea Devils, a race of reptiles in hibernation in a base beneath the sea, who have been awoken by recent work on a nearby sea fort. The Master intends to use his new allies to help him conquer the world!
Jon Pertwee
The Doctor
Roger Delgado
The Master
Katy Manning
Jo Grant
Edwin Richfield
Captain Hart
Clive Morton
Trenchard
June Murphy
3rd Officer Jane Blythe
Donald Sumpter
Commander Ridgeway
David Griffin
Lt. Commander Mitchell
