In the Atlantic, Gestapo agent Gunther Rudehoff is assigned to oversee a group of foreign scientists imprisoned on a German U-boat commanded by Capt. Ganz. En route to Germany, the boat receives orders to torpedo an American tanker and pick up a German spy named George Akton. Aware of the planned attack, Akton dives overboard with a life raft just as the ship explodes in flames. Archie Gibbs, a sailor aboard the doomed tanker, follows and after overpowering the spy, climbs into the raft. After he is picked up by the U-boat, Gibbs assumes Akton's identity and Rudehoff decides to quarter him with the scientists, ordering him to report any pertinent information that he may glean. Escorting Gibbs to the scientists' quarters, Rudehoff introduces him as an American prisoner to scientists Clyde Hamilton, Sigo Van Der Brek, Alfonse Lamont and Biencawicz. An American destroyer has been trailing the submarine, and when it attacks with depth charges, Ganz decides to shoot an injured sailor through the torpedo tube to convince the Americans that they have sunk the U-boat. Disgusted by this display of Nazi brutality, Gibbs confides his true identity to the scientists. Meanwhile, in the control room, Rudehoff and Ganz argue about their priorities. Rudehoff demands that the boat proceed directly to Germany to deliver the prisoners while Ganz insists on attacking a nearby American convoy. When the engine of one of the convoy ships fails, a destroyer is detailed to protect the craft while the rest of the convoy continues on. Spotting the floating body of the half-dead German sailor, the American ship rescues the man who, on his death bed, warns that the submarine is unharmed. After the engines are repaired, the Americans decide to use the ship as a decoy to lure the U-boat. The ploy works, and after the U-boat surfaces, American naval officer Lt. Hagen volunteers to man a small boat and pinpoint the exact location of the Germans. Upon surfacing, the Germans discover the destoyer and decide to attack the vessel with magnetic mines because the fog is too dense to sight it with torpedoes. For the mines to attach to the steel hull, they must be within 500 yards of the target, and so a rowboat of German sailors tows the mines toward the destroyer. When Lt. Hagen and the Americans spot the Germans, a fight ensues, sending the mines adrift. Meanwhile, Gibbs conspires with the scientists to alert the Americans to the U-boat's location. When Gibbs proceeds to the radio room to signal the Americans, Lamont, fearful of being sunk by the destroyer, exposes Gibbs's true identity. As Ganz confronts Gibbs and the scientists in the radio room, a mine drifts into the U-boat and explodes, sending the submarine to the bottom of the sea. All but the radio room floods, and the only survivors are Gibbs, the scientists, Ganz and three German seamen. As the fog lifts, the Americans search for the submarine and find a signal buoy that Ganz has sent to the surface. Having almost run out of air, the survivors realize that the torpedo tube is their only hope of escape. When Ganz decides that the scientists must remain behind, Lamont protests and Ganz shoots him. In the ensuing scuffle, Gibbs assumes command of the ship and dispatches the men to the surface until only he, the injured, unconscious Lamont and Ganz remain. A drawing determines who must stay behind to dispatch the last survivor to the surface, and when Ganz wins, he climbs into the torpedo tube. At that moment, Lamont regains consciousness, knocks Gibbs out, slips him into the torpedo tube and sends him to the surface, leaving Ganz and himself behind to die a watery death.
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