The Sea Devils attack the island prison and rescue the Master, prompting the Doctor to venture underwater to try and contact them.
Quest roles:
Edwin Richfield(Mestor), Terry Walsh(Doran (uncredited)), Pat Gorman(Pilot), Donald Sumpter(Commander Ridgeway), Roger Delgado(The Master), Stanley McGeagh(Castle Guard Drew), Clive Morton(Trenchard), David Griffin(Lt. Commander Mitchell), Colin Bell(Private Bryson), Brian Vaughan(Lt. Commander Watts), Alec Wallis(Warner), June Murphy(3rd Officer Jane Blythe)