AllMovie

The shakedown cruise of the Seaview's sister ship, the Polidor, ends in tragedy when the new ship is lost with all hands. Nelson (Richard Basehart) and Crane (David Hedison) must determine if it was a design failure or some other problem that caused the wreck. The Seaview repeats the dive with two expert psychological observers (Edgar Bergen, Lloyd Bochner) aboard. But they don't know that one of the two guests is an enemy agent who has programmed the release of a fear-inducing gas into the ventilation system -- a gas that turns deadly after eight hours.