Amid rumors of war in Europe, a group of Americans rush homeward aboard a British freighter. On board are Mike Farrough, an American soldier of fortune; John Morgan, a district attorney who has put his office at the service of gangsters; Christine Blaine, Morgan's secretary and sweetheart; Larry Perrin, a fugitive murderer trailing Morgan to avenge a double-cross; Professor and Mrs. Mudge, an elderly couple; and Dr. Behrens, a German doctor. One day out of Liverpool, war is declared, and soon after, the freighter is halted to take on guns. When a German submarine appears, Farrough mans a gun and cripples the U-boat's periscope. Dr. Behrens, torn between ethics and his love for his fatherland, sends signals to the submarine with a shortwave transmitter. As the submarine tracks its prey, Perrin watches for his chance to kill Morgan, while Farrough and Christine fall in love. Discovering that the doctor has been signaling the submarine, Farrough places the transmitter aboard a small boat loaded with depth charges. To decoy the submarine away from the freighter, Perrin volunteers to pilot the boat in a suicide mission, and forces Morgan to accompany him. When the submarine attacks the decoy, the depth charges explode, sinking the sub and saving the freighter at the cost of Morgan's and Perris' lives.