Kowalski's brother Stan is rescued from a wrecked sub by Crane and the crew. But a malfunction on the wreck launches a torpedo that cripples the Seaview and sends her to the bottom.

Stan's cowardly refusal to help the sub with his special diving gear brings morale to a dangerous low, as the air supply runs out. Captain Crane will use the unfamiliar equipment and risk his life in a last desperate gamble to save his ship.