Danish Captain Conrad Veidt and his freighter Helvig are slowed in the English Channel for a routine cargo inspection at a British Contraband Control Port, a tense scene as World War II crawls across Europe despite Denmark’s neutral stance. The encounter sets a grim mood of vigilance and espionage that threads through what follows.
Captain Andersen is granted two shore passes for himself and his First Officer Hay Petrie to dine with Lt. Cmdrs Ashton and Ellis, portrayed by Joss Ambler and Harold Warrender. But those passes—and Helvig’s motorboat—are stolen by two travelers: Mrs. Sorensen, played by Valerie Hobson, and Mr. Pidgeon, portrayed by Esmond Knight. The theft triggers a perilous chase that quickly spirals beyond a simple theft.
Relying on a cut-out newspaper train schedule, Andersen works out that the pair are headed toward London and manages to close the gap on the train. Yet upon arrival in the blacked-out metropolis, he clings to Mrs. Sorensen, barely keeping her within reach as danger looms. He then guides her to the restaurant of Skold’s brother Erik, and later to the home of her aunt, where a Nazi spy ring led by Van Dyne takes them captive. Van Dyne, a figure who already has a troubling history with Mrs. Sorensen and Mr. Pidgeon, proves a keen organizer of the operation.
The spy mastermind—Raymond Lovell as Van Dyne—discovers a hidden note on one of Mrs. Sorensen’s cigarette papers. The message identifies her codename as “M47” and lists the neutral ships under which two German vessels are traveling. Van Dyne’s plan is to swap one of the ship names with that of an American vessel to rile the still-neutral United States and sow confusion on the high seas. The stakes rise as Mrs. Sorensen and Andersen are bound, while the clock ticks on the bigger game of naval diplomacy and espionage.
Despite the tighter grip, Andersen manages to escape and call for help with reinforcements. These arrive in the form of Erik Skold’s staff, who aid in the struggle and help subdue Van Dyne. The tide turns, and Mrs. Sorensen is freed as the immediate danger fades. In the end, the threat is contained, the ruse is thwarted, and Captain Andersen and Mrs. Sorensen resume their voyage, the waves once again bearing witness to the fragile line between neutrality and conflict.
