On the night of 20 Nov 1941, two high-ranking Nazi naval officers escape from a prisoner of war camp on the Canadian border. When one man is shot, he orders the other to go to San Francisco to meet with their confederates. In San Francisco, the escapee runs into harbor detective Sam Dillon, but the officer, not recognizing him, lets him go. The German then goes to "The Schooner Club," a waterfront café where an anniversary party is being held for its Italian American owner, Nick, by his stepdaughter, Gen Rand. The escapee, using the name Bill Flynn, asks Gen to sing "Who Is Sylvia?" in order to signal his cohorts of his arrival. The café's Asian waiter Ito tells Bill to go to Warehouse C on Pier 46, and Gen follows him there. Bill meets with Nick, a hired hand of the Axis powers, and tells the café owner that he is actually Captain Wilhelm Roering. Nick then arranges a meeting between Bill and the head of the local German spy ring. Gen later meets with Sam and tells him about the Nazi's arrival at the club. After Bill takes Gen out to dinner, Nick warns him to stay away from his stepdaughter. Bill then meets with Franz Muller, president of the German American Exporting Co., who offers to put him in command of a captured Japanese ship currently being held in San Francisco Bay. It is Muller's plan to free the ship, man it with German, Italian and Japanese loyalists and use it to raid American vessels in the Pacific. Meanwhile, Sam tells police inspector Alan Davies of Muller's true allegiance, and they scheme to capture the Axis agents that night. Unfortunately, Gen inadvertently tells Nick of those plans, and he warns the spies. Despite this, Muller refuses to pay Nick the $10,000 he is owed unless he agrees to hide some Italian nationalists in his club until they can commandeer the Japanese ship. Using a rowboat, Nick and Muller smuggle the men out of the warehouse, but are discovered by Sam as they make their escape. Nick is wounded, but the spies manage to flee the police in a power boat. After arriving safely at the Schooner Club, Bill visits Gen, who has been informed of his identity, and Sam arrests Bill, but later learns that Bill is actually Capt. William Flynn Hancock, a Canadian intelligence officer. Bill tells Inspector Davies that he impersonated captured German officer Capt. Von Ritter at Roering's prison camp, then convinced the German that he was under orders to accompany him on his escape to San Francisco; when Roering was killed in the escape, he then assumed the dead Nazi's identity in order to discover his mission. That night, however, the real Roering arrives at the Schooner Club, and unsure of his identity, Nick and Ito send for Muller. Gen sends a warning note to her father, but the spies intercept it and capture Bill. Learning that the Nazis plan to abduct Gen as well, Nick goes to her rescue, but is knocked unconscious by Ito after Gen convinces him to turn his employers over to the police. As the spies capture the Japanese ship, Sam and the harbor patrol arrive. The spies are captured, but Nick is killed in the shootout, along with Roering and Ito. Gen is later consoled by Sam, and Bill agrees to marry her after the war is over.