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The Underwater Robot was a giant humanoid automaton controlled by a group of criminals led by an individual known only as the Professor.
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They used the robot to attack cargo ships at sea, living off the supplies they captured and enslaving the survivors to work the engines driving the robot. When the Doctor, John and Gillian arrived at the ocean floor and went exploring, they were captured and put to work in the engine room. By allowing the robot's legs to stop functioning, they escaped into the control room, where the Professor tried to kill them with a speargun. Instead, the Doctor deflected his aim and the spear shattered a window, flooding the robot and freeing the slaves, who could breathe underwater. (TVC: The Underwater Robot)