In London, the Italian gym teacher Enrico 'Henry' Rosseni is having a love affair with his eighteen year-old student Elizabeth Seccles, who is the daughter of the owner of the Catholic School where he works and she studies. His estranged wife Herta Rosseni is the mathematics teacher in the same school. On Sunday afternoon, Rosseni and Elizabeth are in a boat in a grove and the girl glances at a knife and a man hunting down a woman. Rosseni believes she is giving a pretext to avoid him and does not pay attention to her. On the next morning, Rosseni learns that a teenager was murdered in the river bank in the grove and he drives to the location. Inspector Barth goes to the school since the victim Hilda studied there and soon Rosseni becomes the prime suspect since he protects Elizabeth, who could give an alibi to him. When a second student is murdered, Elizabeth recalls that the killer where a black soutane worn by priests. While the police investigate the suspects, Elizabeth is killed and Rosseni decides also to investigate. He teams up with Herta and the find the name of the mysterious Solange Beauregard. They seek her out but what might have happened to Solange?
A teacher who is having an affair with one of his students takes her out on a boat. They see a knife killing on shore. Other gruesome murders start occurring shortly thereafter, and the teacher suspects that he may be the cause of them.
- Enrico and Elizabeth are making out in a rowboat when Elizabeth sits up and thinks she sees a girl running along the shore. She interrupts the kissing session again when she sees a knife being swung. Enrico, thinking she just wants an excuse to stop, angrily rows the boat to shore. Enrico and Elizabeth leave, and a pen is dropped. Enrico, at home with his wife, Herta, hears a radio report of a murdered girl. On his way to school where he works as a teacher, Enrico stops by the crime scene. Inspector Barth is at the school with the headmaster Mr. Leach and some teachers with Enrico walks in late. Enrico leaves, passes by a classroom, and sees Elizabeth (she's a student). He goes to gym class and tells his students that their classmate Hilda has been killed.
Later, Enrico asks Elizabeth what she saw then they were together. She says she saw a shadow like a man and "something funny". Enrico asks her not to go to the police, for fear that they will ask Elizabeth who she was with. At home, and an irate Herta asks Enrico where he was that morning (he had lied by saying that his car and broken down). She shows him the front page of the newspaper, with a photograph of the crime scene and Enrico pictured in the background. Inspector Barth talks to Enrico and asks him how he found such a obscure local in such a short time. Barth is shown a pen that was found on the victim's dress, and he talks to Hilda's parents, who say that their daughter went to confession the afternoon she was killed.
Enrico calls Elizabeth at home and gives her an address for a place to meet. She tells her uncle that she is going to study with Brenda. She rides her bicycle and is followed by someone in a car. Enrico lets Elizabeth into a small apartment and two lovers kiss. Some calls from a pay phone and hangs up.
Enrico is back at home grading test papers, and Herta asks him if he had sex with Hilda. The inspector arrives at the front door, and he slyly places Enrico's pen on the table. Enrico sees it. Herta leaves the room, and Barth states his belief that Enrico was a witness. The teacher admits to being there with Elizabeth and says she claims to have seen the killer.
Janet is at home with the maid when a man, saying he is Helen's father, calls and says that Helen needs to borrow Janet's Italian book. As she goes to the car outside, tape is slapped over her mouth and she is driven away. Janet is dragged out of the car and stabbed to death.
Elizabeth wakes up in bed, thinks back to Hilda running and calls Enrico. Elizabeth runs to her car when Enrico arrives and tells him she remembers that the killer wore a black habit, like a priest. The next day, Elizabeth tells Mr. Bascombe she knows something about Hilda's murder. He leads her to a room of teachers (including Enrico) where she tells of the priest-like pursuer. Afterwards, Enrico thanks the girl for leaving him out of the story. Later, the killer walks into Enrico's apartment and takes the needle off of the record that is playing. Elizabeth is attacked and drowned in the bathtub while taking a bath. Outside, a bearded man sees the killer run away.
Enrico is arrested for Elizabeth's murder, and Barth interrogates the other teachers at the school. At the police station, Barth receives the autopsy report that hair was found under Elizabeth's fingernails which turned out to be dead hair. The report also says that Elizabeth was not sexually assaulted and was in fact a virgin. The bearded man is brought in for questioning and later looks at a line-up of priests and says that the man he saw running away from the building had a beard. Barth is told that, because of the dead hair, it was probably a false beard.
Barth tells Herta that Enrico did not kill anyone and that Elizabeth was a virgin, and the teacher seems happy. Enrico says he wants to find out who killed Elizabeth, and Herta offers to help. In Herta's class, Barth asks the girls if they know a bearded priest. Brenda says that Janet confessed to a priest with a beard at Hilda's funeral. Herta talks to the girls and learns of a secret society. She tells Enrico about the society and says that Hilda and Janet once dated university students, and she has a name.
Enrico goes to see Phillip and asks him about Hilda and Janet; he talks about the girls hanging out with him and his friends. He says that, after a while, they no longer had sex with boys after what happened to Solange, a girl who used to go to the school and has suddenly disappeared.
At the funeral for Elizabeth and Janet, a man hands something to Helen, who passes it on to Brenda. Enrico talks to the two girls and asks about Solange, whom they claim not to know. Bascome wants Enrico about "prying" with the girls.
Herta calls some schools and learns about one Solange that did not return. Brenda leaves a note in Henry's mail. The next day, Herta goes to the school and Enrico goes to the address where Solange once lived' the woman who answers the front door says she did not know the previous owners. Enrico picks up his mail and drops Brenda's note. The killer calls and suggests he take a vacation or he will not see "her" again. Enrico calls the rental agents, but the offices are closed.
Downstairs, Enrico is handed the fallen note, with the name Ruth Holden written on it. Enrico goes to her place and finds a dead dog, a bloody shovel, and Ruth dead with a sickle in her neck.
Herta asks Helen and Brenda if they know Ruth, but they respond negatively. The inspector stops them and says that Brenda's mother told him Ruth was the girl's maid. Brenda says she forgot the woman's real name.
Enrico, lying in the sun with Herta, says he thinks that the killer dressed as a priest and confessed the girls. A quiet girl stands near the couple and runs away and a woman calls after her: "Solange!"
The killer phones Brenda and tells her to go somewhere, and the cops (listening in) set up a trap for the killer at a carnival. Brenda sees Solange on the merry-go-round and takes the mute teenage girl away. A cop dressed as a priest loses sight of the two girls, and they are seen being driven away by another priest. Bascombe comes into the station and says that his daughter, Solenge, is gone, and he shows a picture of her. The inspector tells an inquisitive Enrico that Solange is sick in the head and has experienced "infantile regression". Enrico, Barth, and Herta go to Bascombe's house and the inspector sees a green pin by a photograph of Solange. Solange is in the next room with a restrained Brenda, and the killer tells her to explain everything. Brenda recalls the girls going to see Ruth, and Elizabeth and Hilda trying to talk to Solange out of going in and letting "nature take its course". But the others lead her inside and hold her down as Ruth gets a needle and performs a painful abortion on the pregnant Solange.
Enrico and Herta to go Janet's place, and Enrico looks for her Italian book. He returns to Bascomb's place with Barth and another officer. When the man does not respond, Enrico kicks the door open and shows a second form Italian book (Solange did not get that far in school). Solange is at the door, and Bascombe returns and hugs his daughter. The men talk to him, and Solange pulls Herta away and leads her to a priest's habit. Herta calls Barth, and he and the cops run into the corridor and find the still-living Brenda. In his office, Bascombe pulls out a gun and shoots himself. Barth tells of the girls society (signified by green pins), their sex parties, and Solange's traumatic abortion. Solange sees her dead father and cries.