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The Polidor, another ship launched by the Nelson Oceanographic Institute, is exploring the depths to determine how practical life in the deep water may be. In contact with the Admiral, the crew becomes confused and then incoherent, and the ship slips below the safe line and is lost. Seaview continues with the tests. Unknown to the already stressed crew, one of the visiting psychologists aboard is an enemy agent who has released a fear-causing gas into the air system - the same gas ultimately responsible for the loss of the Polidor. As Seaview plunges into the depths a dangerous panic starts to spread.