During the Second World War, Sir Edward Norton of British Counterintelligence informs politician Goodenough that all he knows about a German spy in possession of vital information is that he or she is on the ship Santa Maria, bound for Lisbon. However, there is some good news; Sir Edward receives a coded message from the ship's captain, informing him that Lloyd Harrigan, one of his most resourceful agents, is also aboard. Despite being a neutral, the ship is torpedoed.
Six passengers make it to a lifeboat, three men (Robert Cooper, Thomas Pettifer and Toni Anzoni) and three women (Gillian Chase, Mrs. Lindley and Diane Winters). They pick up Oberleutnant Hartzmann, the commander of the now-sunk U-boat, and soon after, reach a lighthouse manned by Sutton and his assistant Ernie. Hartzmann manages to send a message before disabling the wireless, and finds a pistol. Holding the others at gunpoint, he informs them that another U-boat will pick him up in about five hours. He also tells them that he sank their ship because he knew that both Harrigan and the German spy were passengers, and that, based on Harrigan's reputation, the spy would not have reached Lisbon. However, he does not know either person's cover identity. Nobody admits to being either agent. Eventually, Sutton obtains another gun and takes Hartzmann prisoner, handcuffing him and locking him in a room. However, someone passes the keys to Hartzmann's handcuffs and the door, and leaves a revolver outside. Hartzmann shoots the wireless Sutton has repaired and takes control again. Then Cooper turns up dead, struck in the head.
Finally, when Hartzmann announces the U-boat has arrived, Pettifer reveals he is the spy. However, Hartzmann then informs him he is Harrigan, not the unfortunate Cooper. British, not German, naval personnel take Pettifer away.
