The Danish freighter Helvig approaches English waters in early 1940 and, in keeping with the needs of British security, it is boarded by customs and naval officials in search of contraband cargo. Her skipper, Captain Anderson (Conrad Veidt), is compelled to ascede to British demands, but dreads the delay, pointing out that the medical supplies in his cargo are vital. Anderssen is a dedicated seaman, all business, even where Mrs. Sorenson (Valerie Hobson), a headstrong passenger, is concerned. Then, on their first night in port, Mrs. Sorenson and a Mr. Pidgeon (Esmond Knight) disappear from the ship with Anderson's landing papers, the captain is in hot pursuit. Forced to join the woman in what seems a mad chase across London by night, he plunges into an Alice-in-Wonderland world of the blacked out city, following a set of clues through the maze of darkened streets and uncover a Nazi spy ring operating out of a basement in Soho. Each also discovers that there's a lot to admire and even possibly to love in the other -- the challenge is for Hobson, who is something other than the divorcee and mother she pretends to be, to stay alive long enough for Captain Anderson to effect a rescue and prevent the German spies from turning the British counter-intelligence effort against the Allies.