Mystery Submarine

Madeline Brenner is questioned by the United States Attorney in New York as treason charges loom over her. The interrogation frames a flashback-driven recount of her wartime actions and the choices she made.

In a Cape Cod flashback, Madeline works as a secretary for Mrs. Weber on the coast and believes her husband died in 1945 when his U-boat, U-64, was sunk. The rumor of his survival begins to pull her into danger. The memory sets the emotional stakes for the current crisis.

A mysterious man, Eric von Molter, approaches Madeline on the beach and claims her husband survived. He suggests she can be reunited with him if she helps kidnap Dr. Gurnitz, a German scientist visiting Mrs. Weber. The proposal sets the plot in motion.

During a yacht cruise, Madeline disables the radio as the U-64 surfaces nearby. The submarine’s crew abducts Dr. Gurnitz while the passengers remain unaware. The kidnapping hinges on Madeline’s cooperation with von Molter.

Onboard the submarine, Madeline learns her husband is dead, though the U-64 survives. The man who recruited her, von Molter, is now in command and used her to obtain Gurnitz. She realizes she has been exploited for the plan.

Von Molter sinks the yacht, killing Mrs. Weber and the passengers to silence witnesses. The brutal act ensures the kidnapping remains secret and demonstrates his ruthlessness.

Captain Elliott and Doctor Brett Young explain that the Navy suspects a renegade submarine base near Belgrana in a South American country. They decide to send Young to investigate, initiating the counter-operation.

Young searches the coast and discovers the cove where the U-64 hides. He is captured, but von Molter buys his cover story and invites him to serve as medic for Gurnitz.

Madeline is confined on von Molter's estate, while the plan to trade Gurnitz on the high seas unfolds. Young tries to warn his superiors and schemes to go to Belgrana for medicine, but von Molter blocks him.

In Belgrana, Young slips a note to a nurse at the clinic seeking help. Von Molter shoots the nurse to remove witnesses and retrieves the note.

Back at sea, Madeline takes a calculated risk and fires a flare to reveal the U-64’s position, following Young’s plan. US naval depth charges respond while von Molter attempts to simulate a sinking by ejecting debris.

Among the debris is a lifejacket with coordinates of the planned handover, written by Young. The Navy uses this clue to set a trap for the impending exchange.

At the meeting point, Von Molter prepares to hand over Gurnitz while Madeline and Young are to be killed. US Navy planes suddenly bomb the U-64, destroying it and saving the day.

In the present, Young argues that Madeline risked her life to save them all and should not be charged with treason. The U.S. Attorney agrees, and Madeline walks away arm in arm with Young.