The Doctor and Jo visit the Master in his high-security prison on an island off the south coast of England. The governor, Colonel Trenchard, says ships have been disappearing mysteriously at sea. The Doctor discovers that Trenchard and the Master are in league to contact the Sea Devils, a race of reptiles in hibernation in a base beneath the sea, who have been awoken by recent work on a nearby sea fort. The Master intends to use his new allies to help him conquer the world!
as The Doctor
as The Master
as Jo Grant
as Captain Hart
as Trenchard
as 3rd Officer Jane Blythe
as Commander Ridgeway
as Lt. Commander Mitchell
as Hickman
as Robbins
as Sea Devil
as Chief Sea Devil
as Clark
as Ldg. Telegraphist Bowman
as Castle Guard Wilson
as Castle Guard Barclay
as C.P.O. Smedley
as Ldg. Seaman Lovell
as Castle Guard Drew
as Lt. Commander Watts
as Walker
