AllMovie

The Seaview is attacked by an enemy submarine while searching for a secret underwater base. Admiral Nelson (Richard Basehart) and Captain Crane (avid Hedison) try desperately to get the stricken ship off the bottom, and Crane must rescue one crewman (Paul Carr) from a repair mission gone awry. While repair crews work to raise the sub, Crane tries to carry out the original mission, taking a diving party out to try and reach the undersea base. Before they can get there, however, they're attacked by a giant sea creature, and must struggle to save themselves.