The Doctor is held prisoner by Trenchard while the Master attempts to contact the Sea Devils.
Quest roles:
Edwin Richfield(Mestor), Terry Walsh(Doran (uncredited)), Donald Sumpter(Commander Ridgeway), Roger Delgado(The Master), Stanley McGeagh(Castle Guard Drew), Christopher Wray(Ldg. Seaman Lovell), Clive Morton(Trenchard), David Griffin(Lt. Commander Mitchell), June Murphy(3rd Officer Jane Blythe)