context: Over the opening credits a man is with two women, all of them nude and on a bed fooling around, while a camera is taking photographs. After the credits, a professor is speaking with his students, and Jane (Suzy Kendall) an American exchange student attending college in Rome, Italy speaks with the professor after class. Stefano (Roberto Bisacco) offer's Jane's friend, Dani (Tina Aumont), a ride home, but she turns him down. She later tells Jane and the man seem to be always following her.Later, Flo (Patricia Adiutori), another one of Jane's friends, is with Sean, making out in a parked car at an overpass. Sean sees someone wearing a ski mask watching them. Sean gets out of the car and runs after the person. Flo gets out of the car, turns on the headlights and looks for Sean. When the headlights are turned off, Flo heads back to the car where the man with the ski mask stabs her in the chest.The next day, Dani shows Carol (Cristina Airoldi) the news of Flo's death. Classes are canceled for the day and Jane goes to see Professor Franz (John Richardson). The two of them have a drink at a local bar and Franz invites Jane to a concert. Across town, Stefano picks up a prostitute, but seems reluctant to do anything with her. When she demands that Stefano pay, regardless of his actions, Stefano hits and chokes the woman. She leaves behind an unsettled Stefano.The next day, Dani watches Carol being taken away by two bikers. They gloved killer is driving around, passing Dani who looks right at the driver. At a hippy party, Carol is kissing the two men and smoking dope, but when one of them tries to further the interaction, she puts her joint out on his chest. Carol leaves and ends up in a marshy area. She sees the masked killer standing a short distance away in the fog and runs away, but the killer catches up with her and knocks her down to the ground. Carol is strangled with a scarf and her head is pushed underwater. When Carol is dead, the killer gets a knife and cuts into Carol's eyes.The next day, the police show the students of the art class the scarf that connects the two recent murders. Dani tells Jane that the two bikers who took Carol and that they had no scarfs. At her house, Dani gets a phone call in which the killer threatens her about her scarf knowledge. Dani's uncle comes home (Jane had seen the man earlier talking to Carol) and suggests that Dani go to the country house in Perugia for a few days to hide out and relax.Meanwhile, Inspector Martino interviews a vender and ask him if he remembers selling a scarf. The man denies any recollection, but after the inspector leaves, the vender gets on a pay phone can tells someone that he wants money to keep quiet about a transaction concerning the scarf in question. A little later, Stefano talks to Dani saying that she "belongs" to him. Dani remembers Stefano wearing the scarf. She gets away from Stefano, calls Jane and asks her to come over.After Dani tells Jane about Stefano, his scarf and the threatening phone call, Jane says she will be coming to the country house a little later then the rest of them. That evening, the "scarf seller" is walking at night with a healthy chunk of cash when he is chased down by a car and crushed against a wall. The killer then takes the cash that he had apparently given the vendor earlier.The next morning, Jane goes to see Stefano and after she lets herself into his house, she sees a doll (recalling the doll flashing in the killer's mind during the murders). A neighbor woman tells Jane that Stefano was not home last night and that he might be out of town.Meanwhile, Dani and her two friends, lesbian couple Ursula and Katia, are traveling by train to Perugia. A friendly doctor named Roberto (having occasionally been seen around the university in Rome) enters their compartment and sits by Dani. The three of them arrive in Perugia and draw some attention with their revealing modern clothing as they purchase goods at a local grocery store for the week they are at a villa over looking the town. That night, Jane arrives outside Perugia in her car at an Esso gas station to ask for directions to the villa and for her car to be worked on. The mechanic says that he cannot work on the car until tomorrow for the garage is closing for the night, so he gives Jane a ride in his car up to the house on the hill. At the house, Jane tells Dani that Stefano probably did not commit any murder. Dani remembers Stefano's scarf design was slightly different for the scarf he was wearing had red stripes on a black background, not the black stripes on a red background as described by the police, but she knows that she saw a man driving a car earlier wearing the scarf but cannot remember who it was.A weird guy from the village arrives at the villa and watches through a window as Ursula and Katia lie on a bed and making out with each other. The weirdo sees the killer with his knife, so he runs and hides, but the unseen killer finds him and finishes him off.The next day, a man comes to deliver some groceries. When Jane runs down the stairs to greet him, she slips and twists her ankle. Roberto visits the house to check on Jane's ankle and Dani remembers him from the train. With Jane disabled, the three other women walk to a spring behind the villa to skinny dip. While swimming, Dani think she sees Stefano and calls out to him, but Ursula and Katia do not see anyone. That evening, the women say goodnight to the bedridden Jane. Downstairs, Dani decides to call Stefano and discovers that the phone is dead. The door buzzes and Stefano is standing in the doorway with his scarf, but he drops dead to reveal the masked killer behind him (clutching onto Stefano's scarf). Dani and the other two women scream.The next morning, Jane awakes up, limps to the door and calls out to her friends. She slowly walks down the stairs and screams when she sees Ursula and Katia dead in the living room, having been stabbed multiple times. Dani appears and reaches out to Jane, but she falls to the floor also from being stabbed and left for dead, but she dies before she can tell Jane anything. When she sees the front door being unlocked, Jane hides. The unseen black-clad killer enters the living room and begins sawing Ursula's corpse with a saw as Jane cringes in her hiding spot behind a door from the sight and sounds of it. Just then, the delivery man arrives with his groceries and leaves when no one answers to his knocking on the front door. After the man leaves, the killer resumes sawing up Ursula, puts her disembodied remains in a plastic bag and walks out. Jane comes out of her hiding place and heads for the stairs, and freezes when the door is opened again but the killer is only throwing in the bread and milk delivery left behind. Finding the telephone dead and the front door locked, Jane goes upstairs, despite the intense pain from her sprained ankle, resting at the top and not realizing that her slippers have come off.Jane goes to her bedroom window and sees the town below. She tries to signal someone using her cosmetics mirror reflecting sunlight. Roberto, still in town, looks up at the house and sees the reflection of the sunlight. He checks a phone directory at a local payphone and calls, but receives no answer. Jane, seeing the killer coming back, quickly cleans up the bedroom and hides all her clothes and other stuff to give the appearance of no one being there. She sees that her slippers are gone and looks out the bedroom door and sees the killer sawing up the dead body of Katia. Just then, the gas station owner arrives with Jane's car and knocks on the front door. With the killer again freezing and Jane unable to make a sound out of fear of being discovered, the gas station man leaves. As Jane goes to the window to try to signal a warning, she knocks over a chair. The killer comes upstairs forcing Jane to hide behind a drawer. After looking around the room and finding no one or any belongings, the killer then locks the bedroom door, with Jane still inside and leaves the key in the door. The killer then finds Jane's slippers on the steps, but he instinctively picks them up and throws them onto a bathroom floor. Jane is now locked in her own room while the killer goes back downstairs to resume sawing up Katia and Dani's dead bodies and leaves with them to bury them in a field a short distance away. When he is finished with burying the bodies of Stefano, Ursula, Katia and Dani, the killer then cleans up the house, washing away the blood and leaves while Jane watches, but she is still locked in her bedroom.At night, the grocery man tries to convince a group of local men that there are four women at the house as opposed to three the men had seen arrive. The killer is nearby and overhears the men. At the same time, Roberto goes to the gas station to get fuel before driving away and sees Jane's car there. The gas station manager tells him that although no one answered the door when he went to drop off Jane's car earlier, he also says that he did not see any of the other women leave.Back at the house, Jane, still locked in her bedroom, slides a newspaper under the door and pushes the key through the keyhole. The key misses the paper, but the killer graciously places the key on the paper for her. When Jane retrieves the key and finally gets the door open, Franz grabs her. Both of them are surprised to see each other and Franz tells Jane that "he had to do it", and that the women "were only dolls".A flashback shows a young Franz with his brother and a girl, and his brother, wishing to see the girl in an intimate ways, goes for her doll at the edge of a cliff and falls to his death. This made Franz see women who bring nothing but bad things for men. Franz also tells Jane that Flo and Carol seduced him and blackmailed him (they were the three people in the beginning), and they he had to kill Dani because she would have eventually remembered him and his scarf. Franz says he never wanted to hurt Jane, but since she knows that he is the killer, he must kill her to keep his secret intact.Just as Franz begins to strangle Jane with the red and black scarf, Roberto arrives and he and the crazed professor struggle which leads out the front door and into a barn. The struggle leads to the edge of a cliff that the barn sits right beside. Jane stumbles outside and hears someone fall. She watches warily as Roberto returns to her claiming that Franz is dead and the two of them leave to get the police.
question: Who does the killer blackmail for his silence?
answer: Franz