Doctor Who and the Sea-Devils

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?