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In the third episode of the six-part story "The Sea Devils," the Doctor (Jon Pertwee) and Jo (Katy Manning) have been called in to investigate the disappearances of several military vessels on the high seas. It isn't long before both of the time travelers are attacked by the Sea Devils, a race of bipedal earth reptiles who have been summoned from the depths by the Master (Roger Delgado) in his latest bid to destroy the human race. Much of this episode was filmed on location, with full cooperation from the Royal Navy. Originally shown on March 11, 1972, "The Sea Devils, Episode 3" was written by Malcolm Hulke.