Robert Crain is a wealthy German engineer and pacifist who fled to India under a fabricated Swiss identity after being conscripted into the Wehrmacht at the start of World War II. He is blackmailed by Colonel Statter into taking part in a high-stakes plan to seize a shipment of rubber, a scarce resource crucial to both sides’ war efforts, that will soon be carried by the German merchant ship Ingo from Japan to Nazi-occupied Bordeaux. With forged paperwork and a cover story—he poses as Standartenführer Hans Keil, a senior SS officer needing to return to Germany—Crain must use his engineering skills to disable the scuttling charges that all German merchant ships are ordered to set in order to prevent cargo from falling into Allied hands.
Aboard the Ingo, the captain, Captain Müller, comes across as a principled, humane German whose beliefs sit uneasily with the brutal dictates of the Nazi regime. The first officer, Kruse, is a fanatical Party loyalist who keeps a tight lock on the captain and the crew, a varied mix of Nazi loyalists and labor-conscripted prisoners pressed into service to fill manpower gaps. Crain quietly begins locating and disarming the charges, while trying to stay one step ahead of suspicion. To mask his movements, he leverages the charges to convince Kruse that Crain is secretly safeguarding the ship from a hidden saboteur, and he sows seeds of doubt about the captain’s loyalties.
Complications intensify when one of the prisoners attempts to attack Crain, prompting him to enlist that inmate’s help in a plan to let the Allies seize the ship, and to win Mueller’s cautious trust by persuading him not to fire the scuttling devices when the Ingo is nearly torpedoed by a Japanese submarine, its disguise under a British merchantman complicating matters further.
The submarine carries two German naval officers, several American prisoners, and Esther, a young German-Jewish woman who has endured rape and torture at the hands of her captors. As the ship meets the submarine for a transfer of prisoners, Esther’s defiance remains unsoftened by fear, challenging every German she encounters, including Mueller and Crain. When Mueller is alone with her, he reveals that he intends to help her escape once they reach Europe, but Kruse learns she is Jewish and the plan becomes dangerous to everyone aboard.
Crain’s claim of SS credentials comes under scrutiny when the two naval officers radio Berlin, forcing Crain to push forward his mutinous scheme within a tight 24-hour window. Mueller, growing increasingly uneasy as his anti-Nazi beliefs surface, refuses to become a traitor and resists Kruse’s commands. Anticipating exposure, Crain covertly coordinates a mutiny that would require the aid of the American prisoners, but Esther’s consent to join hinges on a disturbing demand she makes and Crain’s willingness to fulfill.
Just as Kruse acts on the submarine’s broadcast, the mutiny is quickly crushed and the mutineers are corralled at gunpoint. Esther is cast over the deck, and when Kruse tries to force her submission, he is shot by her. Mueller insists he is no traitor, while Crain evades pursuit long enough to detonate the remaining charges Crain had not yet neutralized. The survivors abandon ship, and the anti-Nazi German sailors ensure Kruse’s death by drowning as they watch. In the chaos, barrels of lard rupture in the hold, expanding to form a temporary seal that slows the vessel’s sinking. Crain asks Mueller to radio for Allied rescue, and the captain complies, leaving Crain surprised by the rescue signal that follows.
