Novelist-turned-soldier Edgar Brodie (John Gielgud) is recruited by British intelligence during World War I to ferret out a mysterious German spy and eliminate him. Brodie is given a new identity by his handler, R, and teamed with two professional agents: an amoral, but amusingly deadly assassin known as The General (Peter Lorre) and Elsa Carrington (Madeleine Carroll), a beautiful blonde who will pose as his wife and cover for his new identity. After they mistakenly target an innocent old man as the operative and The General cold-bloodily murders him, both Edgar and Elsa question the morality of the mission, especially when The General only finds amusement in the blunder. When they eventually discover the spy's true identity, Elsa is determined to stop her two fellow agents from fulfilling the mission.