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.

Someone we hear talking — but whom we do not see — speaks of a project which describes the four key moments of love: meeting, physical passion, arguments/separation and making up. This project is to be told through three couples: young, adult and old. We do not know if the project is for a play, a film, a novel or an opera. The author of the project is always accompanied by a kind of servant. Meanwhile, two years earlier, an American civil servant meets with an elderly French couple who had fought in the Resistance during World War II, brokering a deal with a Hollywood director to buy the rights to tell their story. The members of the old couple's family discuss heatedly questions of nation, memory and history.