Aired Feb. 26 - Apr. 1, 1972
Written by Malcolm Hulke
Directed by Michael E. Briant
Traveling to England's southern coast, the Doctor and Jo arrive at the maximum-security island prison housing the Master. There they meet the warden, Colonel Trenchard, who informs them that the Master has been a model prisoner. Meanwhile, he also tells them about the mysterious disappearances of several ships off the coast.
Intrigued, the Doctor heads to a nearby naval base to investigate. The commanding officer, Captain Hart, informs him that the incidents have all occurred near a sea fort scheduled for demolition. The Doctor and Jo travel to the fort, and find that the men there have been attacked by what they call Sea Devils, whom the Doctor deduces are an amphibious offshoot of the Silurians.
While the Doctor again tries to find a peaceful solution, his efforts are thwarted by the Master, who has convinced Trenchard to help him steal equipment from the naval base in an attempt to revive the Sea Devils and foster a conflict.
Once the Sea Devils capture the naval base, the Master has them take the Doctor to their undersea control centre to assist him in reviving the rest of their people. When it becomes clear that, due to misunderstandings and the Master's manipulations, peace will not be possible, the Doctor is forced to modify the machine and rig it to explode. The Time Lords manage to escape just as the machine detonates and the base is destroyed.
The Doctor and the Master are rescued from the sea by the Navy, but in the confusion, the Master escapes to wreak havoc another day.