After their mother's death, Élisabeth and Paul, orphan brother and sister left to their own devices and bound to each other by an exclusive love, live together in their large Parisian apartment. They invent an imaginary universe governed by cryptic symbols. Their bedroom is a sanctuary where a "treasure," embued with a meaning that only the siblings understand, has pride of place. Élisabeth encounters Michael and marries him. But the following day he dies during an accident before the couple's marriage is consummated. Élisabeth inherits Michael's fortune, including a huge town house where Paul comes to join her with their precious treasure. Gérard, a friend of Paul, and his friend Agathe, who strangely resembles Dargelos (a teenage student who Paul idolizes), soon come to live with Élisabeth and Paul. But when Élisabeth realizes that love is growing between her brother and Agathe, like a Greek divinity, a kind of Fate, she weaves a Machiavellian web so that her brother cannot escape her.