 Goro and Juzou Oki set off in an exploration submarine to investigate the attacks in the Japanese Trench; soon later, their vessel is attacked by a mysterious submarine hiding in the Trench.The surface survey ship loses contact with the survey submarine, and two spies storm her command bridge, taking the crew hostage.The spies are overpowered as the mysterious submarine attempts to torpedo the survey ship, and Susumu manages to destroy the incoming torpedo with his rifle.