1940. Pierre Sauvageau , 22, wants to join the army but he must take care of his twin sister Berthe, paraplegic from birth. This closeness awakens Berthe's sensuality who tries to seduce her brother. Pierre rejects her advances, but when he falls in love, he is haunted by the fantasy of his sister. He would like to get rid of it, but the fantasy of Berthe is very persistent.