Who is Joe? She has little history except for her erotic history, little temperament apart from undifferentiated lust. Yet she’s not one of those passive de Sade girls who get hauled off to a château in the forest to be repeatedly violated by lustful men; nor is she like their descendant in “The Story of O.” On the contrary, Joe chooses, she demands, she arranges. In the first part of “Nymphomaniac,” she sleeps with dozens of men, and finally reunites with her first partner, Jerôme (Shia LaBeouf), whom she has come to love. Initially happy with him—she gives birth to his son—she nevertheless announces, in despair, that she has lost any sensation of sexual pleasure. In Volume II, Jerôme urges her to get her groove back by experimenting, and, in a comic episode, she takes up with two African brothers she spies from her window. Then, ever more desperate, she flings herself into masochistic rituals with a professional sadist (Jamie Bell), which is not at all comical—the sessions are re-created in excruciating detail. (Von Trier seems to be saying, “You’re fascinated? Well, stomach this.”) Eventually, Joe falls into bed with a young female protégée (Mia Goth).