Maria is a prostitute who has a very obsessive relationship with her younger teen-age sister, Caterina. Maria behaves as Caterina's mother, sister and lover and she only lets her out of the apartment to go to dance classes. Caterina loves Maria.
She happily accepts her state of "imprisonment" until the day she falls in love with Giovanni, the caretaker of the dance school. When Maria discovers their affair she develops a homicidal fury.