A thrilling wartime espionage drama in which the respected naval commander Richard Heritage (James Mason) is duped by Mary, an attractive, enemy agent who plants false naval movement orders on him. As a consequence of these false plans, a merchant ship is lost to enemy submarines and Heritage is court-martialled for disobeying orders before being found guilty and summarily dismissed from the Navy.
Following his return to civilian life Heritage endeavours to prove his innocence by tracking down the beautiful agent who double crossed him. Working on a tip-off that she is at a remote house on the coast called Orchard Cottage, Heritage visits the house during a stormy night only to discover Mary’s dead body in one of the bedrooms. When Laura Verity (Joyce Howard) enters the house a few moments later, she presumes that Heritage is the murderer and flees to call the police. But when the dead body disappears, Laura and Heritage begin to realise that they are just pawns in a dangerous chess game played by Christopher Child (Tom Walls), the head of a Nazi spy ring.