Detective Marco is assigned to investigate a murder that has occurred at an exclusive boarding school for adolescent girls. The victim is a popular, wealthy girl found strangled in her bed. The school director tells Marco she expects him to find the killer outside of the school, and she bristles at his insistent interrogations of the school staff and the students. Marco learns that two sadistic games were played the night of the murder and that the victim had been tied up. There are a number of suspects, including a teacher with an unnatural affection for one of the girls; a suspicious Spanish gentleman at the town-inn, and some of the pupils themselves. The dead girl's best friend gives the detective the victim's letters and some films. At a showing of the films, there is a sequence where an elderly, mild-mannered teacher is shown kissing with the disreputable town photographer.