Au Royaume des Cieux takes place in in a dreadful girls' reformatory. A couple of lovers will try to escape from this living hell.

A series of crimes is being perpetrated around a famous singer. Strangely, during each of the crimes, the artist's voice is heard humming his favorite song. Suspicions lead to a chase through a nocturnal Paris, ending in a disused theater where the assassin has lured his final victim.