Champ Larkin, an embittered prisoner of Alcatraz Penitentiary, feels little empathy for his country's wartime plight. During a blackout one night, Champ uses the cover of darkness to escape from "the rock," and is accompanied by his cellmate Jimbo. Buoyed by a floating crate, the fugitives are swept along by an ebb tide to an offshore lighthouse inhabited by Captain Porter, his daughter Anne, radio operator Paul Brenner and Stormy. Once ashore, the convicts commandeer the lighthouse and take its residents prisoner. The sudden appearance of the fugitives interferes with the plans of Brenner, who, as a covert Nazi agent, has been sending coded messages between a spy ring in San Francisco and a U-boat anchored off the coast. By exchanging double reverse acrostic puzzles with an onshore radio operator, Brenner sends a message to the spies, notifying them that he will pick them up in the lighthouse boat and deliver them to the submarine at one o'clock that night. Jimbo, who is a fan of acrostics, becomes suspicious of Brenner's coded messages. As Brenner anxiously awaits his appointment, the convicts decide to seize the lighthouse boat and escape to the mainland. When Brenner tries to commandeer the boat for himself, Jimbo mortally shoots him and questions why the radio operator would risk his life for the boat. Meanwhile, on shore, the German spies--Fritz Weinermann, the Baroness and Max--become concerned about Brenner's tardiness and decide to visit the lighthouse. Upon arriving there, Fritz, thinking that Champ is Brenner, reveals their mission. After locking Stormy and the Porters in the cellar, Champ and Jimbo discover that the Germans are carrying a secret plan that details the destruction of San Francisco. When the spies offer the fugitives $25,000 and transportation to South America in exchange for the plan, Champ, declaring that he is "a man without a country," accepts the bargain. As Champ and the Germans negotiate upstairs, Captain Porter picks the lock in the cellar and after escaping, locks Champ and Jimbo in the cellar. When the Germans wrest control of the gun from the captain, Champ hides the plans in the generator. Becoming impatient when the U-boat radios that they must board immediately, Fritz theatens Anne's life unless Champ returns the documents. Although Anne is his daughter, the captain proclaims that he would sacrifice her life for the security of his country. The captain's words convince Champ that the war is his, too, and he and Jimbo overpower the spies and send Stormy for help. When Jimbo is wounded in a fight with Max, Champ holds the Germans at bay while the captain climbs into the light tower and signals the location of the submarine to the mainland. Champ is knocked unconscious after subduing the Germans, but in a half-conscious state hears Anne promise to wait for him to complete his prison term. After the air corps bombs the submarine, Champ and Jimbo return to Alcatraz, where the warden implies that they will receive a reduced sentence for their act of patriotism.