Parts: 6
Writer: Malcolm Hulke
Airdates: 26 February 1972 - 4 April 1972
Production Code: LLL
Megalomaniac of the Week: the Master
Creature Feature: Sea Devils
Story: The Doctor becomes intrigued by reports of several ships being lost at sea, so he and Jo pay a visit to the Master in his seaside prison, then slip away to do some investigating. Their trail leads them to an abandoned sea fort, manned only by two men...and under siege from mysterious creatures that have risen from the depths.
While the Doctor and Jo are fighting off "sea devils", the Master is also investigating the missing ships from his prison cell. His incarceration, it seems, is more for appearances than punishment, and the Master fully intends to make use of his freedom by establishing contact with the undersea creatures.
When the Sea Devils make themselves known above the water, the Doctor realizes that they are the same species as the Silurians - and like their cave-dwelling relatives, the Sea Devils wish to reclaim the Earth from the apes which have taken it over. Can the Doctor persuade the Sea Devils to give mankind a chance, or will the Master convince them that war is the only solution?