Madeline Brenner, Märta Torén, a German-born American, sits in a New York courtroom facing treason charges as investigators weigh her involvement in a dangerous conspiracy.
On Cape Cod, she recounts her past work as a secretary for a wealthy woman, Mrs. Weber, Katherine Warren. She believes her husband died in 1945 when his U-boat, the U-64, was sunk, leaving her with unanswered questions and a growing sense of unease. The story pivots when a mysterious man on the beach, von Molter, Robert Douglas, approaches and claims her husband is still alive. He suggests that a reunion might be possible if Madeline helps kidnap Dr. Adolph Guernitz, a renowned German scientist visiting Mrs. Weber.
During a yacht cruise arranged by these forces, Madeline disables the yacht’s radio just as the U-64 surfaces, making Guernitz vulnerable to capture. The submarine’s crew seizes Guernitz, and aboard the vessel Madeline discovers a shattering truth: her husband is dead, though his submarine has somehow survived. The man who recruited her, von Molter, is now in command and intends to use Madeline to secure Guernitz for a higher-stakes arrangement. The plan appears to hinge on a precise exchange at sea, with von Molter determined to ensure the handoff goes off without a hitch.
The group’s scheme begins to unravel as Captain Elliott and Doctor Brett Young, Macdonald Carey, testify about the Navy’s suspicions. Elliott explains that the Navy traced the wreckage to a secret base near the fictional Belgrana in a South American country and deduced that von Molter’s renegade submarine might be involved. Young, a skilled spy, recounts his own painstaking search along the coast to locate the hidden U-64 and his perilous capture by von Molter’s men. Despite the risk, Young’s cover story holds, and von Molter stages his operation by inviting Young to remain aboard as their medic for Guernitz.
Back at the estate, Madeline begins to feel increasingly like a prisoner, while von Molter remains determined to win her trust for the sake of the mission. Young schemes to warn his superiors, hoping to slip messages through the chain of command, but von Molter intercepts every attempt. The plan then shifts toward Belgrana, where Young pushes a note to a nurse at a remote clinic in an effort to spark outside help. In a ruthless move, von Molter shoots the nurse so there will be no witness to the note’s contents, and he seizes the opportunity to keep the Medical information within reach.
At sea, a US Navy ship finally catches up with the U-64 and begins to close in. Madeline, acting on Young’s guidance, fires a flare to expose the submarine’s location—an act that puts her own safety at risk but also signals the trap to the pursuing forces. Von Molter tries a deceptive ploy, ejecting debris from the torpedo tube and even using a lifejacket with coordinates scribbled by Young to mislead the Americans about the meeting point. The sailors press their advantage, and the chases and countermeasures intensify as depth charges rip through the water, threatening to overwhelm the submarine.
When the stage is set for the final exchange at sea, von Molter insists he can complete the deal if he is back within ten minutes, but the Navy refuses to release him. The U-64 becomes the target of a determined air assault as a squadron of US planes emerges, delivering a decisive blow that saves Guernitz, Madeline, and their allies and deals a crushing setback to von Molter’s operation.
Back in the courtroom, Young argues that Madeline risked her life to protect them all and should not be charged with treason. The U.S. Attorney relents, recognizing her courage and the complexity of the situation, and the case closes with Madeline and Young walking away together, arm in arm, their bond sealed by shared danger and mutual respect.
