The Doctor is taken to the Sea Devils' underwater colony where he tries to make peace with the creatures, only for the navy to attack the base.
Quest roles:
Edwin Richfield(Mestor), Pat Gorman(Soldier), Donald Sumpter(Commander Ridgeway), Roger Delgado(The Master), Peter Forbes-Robertson(Chief Sea Devil), Norman Atkyns(Rear Admiral), Christopher Wray(Ldg. Seaman Lovell), David Griffin(Lt. Commander Mitchell), Colin Bell(Private Bryson), Rex Rowland(A / B Girton), Martin Boddey(Walker), Alec Wallis(Warner), June Murphy(3rd Officer Jane Blythe)