In a Parisian apartment window, a woman and her husband can be seen battling an intruder. Under the eyes of the crowd below a fatal shot is fired. The police rush to the scene, but the murderer has vanished into thin air. Six more locked-room crimes follow, and the authorities are baffled. But private detective André Brunel is on the case: these seemingly insoluble mysteries are connected, and it's up to him to piece the puzzle together...