It is November 1939: the Phoney War-stage of the World War II. Denmark is still neutral, but (Danish) Captain Andersen (Conrad Veidt) and his freighter Helvig are stopped in the English Channel by Lt. Commanders Ashton (Joss Ambler) and Ellis (Harold Warrender) for a cargo inspection in a British Contraband Control Port. He receives two shore passes for himself and his First Officer Axel Skold (Hay Petrie) to dine with Ashton and Ellis, but the passes (and Helvig's motorboat) are stolen by passengers Mrs. Sorensen (Valerie Hobson) and talent scout Mr. Pidgeon (Esmond Knight). From a cut-out newspaper train schedule, Andersen is able to figure out they are taking a train to London and catches up with them; but, when the train arrives in the blacked-out metropolis, he is only able to hold on to Sorensen. He invites her to dine at the restaurant of Skold's brother Erik (also Hay Petrie). Then she takes him to the home of her aunt, where they are captured by a Nazi spy ring led by Van Dyne (Raymond Lovell), a man Sorensen has already had unpleasant dealings with in Düsseldorf, Germany. Van Dyne knows Sorensen and Pidgeon are British agents. Van Dyne finds a message hidden on one of Sorensen's cigarette papers, identifying her as "M47" and listing the names of neutral ships under which two German vessels are traveling. He decides to replace one of the names with that of an American ship to cause trouble, the United States being neutral at this time. Sorensen and Andersen are tied up, but the captain manages to escape. He brings back reinforcements in the form of Erik Skold's staff and is able to free Sorensen and knock out Van Dyne. With everything cleared up, Andersen and Sorensen resume their sea voyage.