While shooting a movie in Brazil, Francesca, an actress, decides to finally sort out a dark chapter of her youth, and to visit her sister Paola in Rio de Janeiro with whom she has been out of touch for more than twenty years. That was when their parents separated, obliging the two children to follow suit. The two sisters are rather distant with one another at first, but gradually grow closer, and Francesca finds out more about her father’s suicide: he had apparently been abandoned by his mistress of many years just beforehand. The intimate relationship between Francesca and Paola is only of short duration, however: when Francesca accompanies Paolo to a party held by her boyfriend Massimo, she and the good looking young man instantly feel a powerful sexual attraction. Paola suddenly starts a fierce quarrel in public. A short while later someone makes an attempt on Francesca’s life, and the two sisters then have a reconciliatory talk. The very next morning, however, Paola is found dead. Inspector Troyberg, a friend of Paola’s, takes over the murder investigation. At her sister’s graveside Francesca meets Greta, her father’s former mistress, who tells her that Paola was schizophrenic, and had undergone psychiatric treatment for a considerable length of time. She had been incurably in love with her father, and had killed him in a moment of intense jealousy, before faking his apparent suicide. Before Francesca can find out any more, Greta is murdered. Francesca then discovers that her sister had been leading a double life as a prostitute named ‘Butterfly’. Francesca finds a video cassette of Butterfly at the apartment of Massimo, with whom she is now deeply in love, and now suspects him. In despair she runs off to see Troyberg, but the police inspector suddenly appears to be totally crazy: he announces to her that it was he and Paola who killed all the others. The inspector had been an ardent admirer of Paola, but when he discovered that she was expecting a baby he couldn’t match that with the image he had of her as the prostitute ‘Butterfly’ he loved so much… After making this confession, Troyberg attacks Francesca but she manages to overpower the madman with her last remaining strength. The nightmare is finally over: overjoyed, Francesca puts her arms around Massimo, who will help her get over all the terrible events.