Whilst visiting the Master, who has been exiled to a luxurious castle prison on a small island, Doctor Who and Jo Grant learn that a number of ships have vanished in the area. Whilst investigating these mysterious disappearances, Jo and the Doctor are attacked by a Sea-Devil, one of a submarine colony distantly related to the Silurians. Soon they discover that the Sea-Devils plan to conquer the Earth and enslave humanity, aided and abetted by the Master. What can Doctor Who do to stop them?