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The Seaview is assigned a mission in the Aegian Sea, to investigate a series of research ship disasters, and an attempt to destroy the sub. Captain Crane (David Hedison) goes undercover in a small seaside village and finds that someone is prepared to kill anyone who asks (or answers) too many questions. He and Nelson (Richard Basehart) follow the trail to Zeraff (Hurd Hatfield), a mad inventor who has built a huge and powerfully armed undersea installation and plans to rule the world, even if it means destroying a good portion of it in the process.