Doctor Who and the Sea-Devils by Malcolm Hulke, 1975 (NM)

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 sub-marine 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?

2nd Edition, 1975