alpertuis is the name of an old, rambling mansion which is in reality a labyrinth where characters from Greek mythology are imprisoned by the bedridden Cassavius (Welles). He manages to keep them (as well as his nephew and niece) prisoners even after his death, through a binding testament. As Jan, the nephew, (Carrière) unravels the mystery, he discovers that he cannot escape the house because Malpertuis is far more significant than he was led to believe.

Overview from themoviedb.org