Doctor Who and the sea-devils

When the Doctor and Jo Grant visit the Master in the island prison to which he has been sentenced for life, they learn that a number of ships have mysteriously disappeared in the area. While investigating, they are attacked by a Sea-Devil, one of a submarine species related to the Silurians. The Doctor uncovers a plot to conquer the Earth and enslave the population. His attempts to prevent its success are frustrated because the authorities refuse to believe that the Sea-Devils are not a figment of the Doctor's imagination or that the Master is not to be trusted... -- Cover description.