New York City. An old man walking his pet dog in waste ground near the foot of the Brooklyn Bridge is horrified when his dog retrieves a decomposed human hand. It is identified by the police as belonging to Ann-Lynne, a local prostitute. Lieutenant Fred Williams (Jack Hedley), the burned-out police detective investigating the murder, interviews the young woman's nosy and obnoxious landlady, Mrs. Weissburger (Babette New), who tells the police detective that during her daily spying and eavesdropping on her tenants, she overheard the girl last week over the phone arranging to meet a man who spoke with a strange, duck-like voice.
Meanwhile, a young woman (Cinzia De Ponti) rides her bicycle down Manhattan to the Staten Island Ferry at Battery Park. After an altercation with a boorish motorist driving a red Volkswagen, whose car she accidentally scratched riding her bicycle, she rides onto the boat with the man yelling misogynist slurs at her. When the ferry is underway, the young woman sneaks into the car-bay and begins vandalizing the man's car, but she is interrupted by an unseen figure. Introducing herself as Rosie, she tries to engage the man in a conversation. But the figure adopts a grotesque 'Donald Duck' voice and brutally murders her with a switchblade knife, stabbing and disemboweling her, and leaving her body to be discovered when the ferry docks at Staten Island. At the morgue, Lt. Williams talks to Barry Jones the pathologist (Giordano Falzoni), who believes he recognizes the style of the killing and links it to Ann-Lynne, as well as a similar case in Harlem the previous month.
Having informed the press that a serial killer is at large, Williams is visited at the station by New York's chief of police (Lucio Fulci). Williams' skeptical superior tells him not to make any further public announcements about the case to avoid starting a city-wide panic. Soon after the police chief leaves, Williams is notified that a man "sounding like a duck" phoned while he was out at the press conference wanting to speak with him. Williams travels to Columbia University where he meets with a brilliant young psychotherapy professor named Dr. Paul Davis (Paolo Malco) for help in creating a profile of the killer. He is surprised that the young genius not only agrees to help with the case, but has already anticipated his motive for wanting to speak with him.
That night in New York's red-light district, Jane Lodge (Alessandra Delli Colli), an attractive, well-dressed woman in a chic raincoat and derby hat, attends a live sex show. She is recording the simulated moans and groans of the two performers with a pocket tape recorder. A scruffy, dangerous looking man (Howard Ross), sitting a few rows behind her, and with two fingers missing from his right hand, observes what she's doing. After the show has ended, the female performer (Zora Kerowa) retires backstage to her dressing room only to find it totally dark. Hearing a noise, she opens a closet door and is brutally attacked by the maniac, who disembowels her by shoving a broken and jagged liquor bottle from her crotch to her abdomen. Later that night, at the home of Kitty (Daniela Doria), a prostitute regularly visited by Williams, he receives a taunting phone call from the duck-voiced killer saying that he has killed again. The detective is perplexed; how does the manic know hes there?
The next day, Jane shows her latest tape recording to her husband Dr. Lodge (Laurence Welles), who has agreed to support their open-marriage. Jane goes to a bar in a rough neighborhood where she's approached by two rough, Hispanic bar punks who proceed to fondle and sexually humiliate her right at the bar. After allowing herself to be taken advantage of, the emotionally troubled Jane runs out of the bar and drives away.
That night, Fay Majors (Almanta Keller), a casually dressed young woman is riding alone on a late-night subway train when she gets menaced by the same ominous man from the porno theater. Fleeing from the perceived threat, she runs off the train, through the deserted subway station, and onto the street where she gets attacked in a dark alley by the quacking maniac, who brutally stabs her in the leg and slashes her hands and arms as she tries to defend herself. Limping away, Fay stumbles through a doorway into a seedy apartment building where she closes and locks the door behind her so the killer won't follow. Fay passes out from the loss of blood, and then realty and illusion blur: Fay is sitting alone in a dark movie theater watching cartoons when she attacked and killed by a different, handsome young man who slashes her neck with a straight razor. Fay wakes up the hospital the morning after when the same man visits her in her room. The man is revealed to be her physicist boyfriend Peter Bunch (Andrew Painter), who is relived that she has survived the attack. Lt. Williams and Dr. Davis visit Fay where she tells them about her attacker who was missing two fingers from his right hand. Williams and Davis both conclude that this is the killer since all forensic evidence points to the killing being left-handed.
Somewhere in night-time New York, the owner of his mutilated right hand picks up Jane and takes her to a sleazy hotel room for sex. He ties up the semi-nude woman to the bed. The S&M game she has willingly begun turns nasty when he begins to beat her without provocation. Then, the man turns up the radio really loud, and makes a muttered phone-call, describing the bound woman to someone on the other line as "she's right up your perverted alley." A little later, while the man sleeps, Jane overhears a radio DJ describing the killer, whom the press has now dubbed, 'the New York Ripper' and missing two fingers from his right hand. Jane carefully and quietly unties herself from the bed and flees into the hotel hallway, only to be killed by the real New York Ripper who stabs her to death in ultra-gory fashion. Williams arrives at the scene of the crime where the police find Jane's tape recordings of the sex shows and of her 'master.' Learning from witnesses, Williams discovers that the identity of the man is Mickey Scellenda, a Greek immigrant with a history of sexual assault and drug abuse. Williams and the police step up the search for Scellenda after raiding his apartment, finding photographs of most of the Ripper victims and huge stashes of pornography. Williams also pays a visit to Dr. Lodge to inform him of his wife's murder. Dr. Lodge tearfully defends his open marriage which gets him a sneering response from the 'moralistic' Williams.
Meanwhile, Dr. Davis begins to express doubt to the killer's identity for Mickey Scellenda is only a petty criminal with a low intelligence quota, not the high intelligence that Davis has created in profiling the New York Ripper. Davis then buys a gay porno magazine at a local news stand (giving away his repressed homosexuality), and then pays a visit to Peter and Fay at their pleasant, well-appointed row house to ask them more questions about Fay's experience. Something about their story arouses his professional suspicious. That evening, after Peter goes out, Fay is attacked in their house by Scellenda who breaks in trying to kill her. But she is saved when Peter returns, and the man flees.
A few days later, Williams gets another taunting phone call from the New York Ripper, who wants to "dedicate a murder to him." Williams and the police put a trace on the line and race to the location, only to find that the killer has set up a two-way radio to a remote phone booth, while he, is at that moment, in the home of Kitty, brutally torturing her by slowly running a razor over her face. Williams races to Kitty's apartment, but is way too late as the killer has fled, leaving behind Kitty's horribly mutilated body to be discovered.
A while later, the dead body of Mickey Scellenda is found having committed suicide from self-suffocation. When Dr. Barry Jones informs Williams that Scellenda was dead for the last eight days, four days before Kitty's murder, Williams finally realizes that they have been tailing the wrong man. Williams relays this to Professor Davis, who is both delighted and disappointed with the news. Davis explains that with Scellenda eliminated as a suspect, his original idea to the killer's identity is confirmed; a deeply disturbed person, with a high IQ, who blames women for his or her troubles, and used Scellenda to pick out his victims. When Davis explains his suspicious about Peter and Fay, Williams decides to have them followed.
Fay is shown visiting a hospital where Peter has a child from a previous marriage, a little girl named Suzy, who is dying from a rare bone disorder that has led to the amputation of her left arm and right leg. But is the killer Peter or Fay? Visiting the hospital, Williams and Davis observe little Suzy in her hospital bed and decide to race over to Peter and Fay's place to arrest both of them. At Peter and Fay's house, one of them gets a phone call from a duck-voiced person, while the other one overhears. When Peter goes into the kitchen for dinner, Fay has disappeared. Going upstairs to Suzy's bedroom, Fay jumps out of the darkness at Peter stabbing him with a kitchen knife. Suddenly, Peter rises, quacking like a duck, and struggles with Fay in which they both tumble down the stairs. Just as Peter grabs the knife away from Fay and about to stab her, Williams runs in and literally blasts Peter's face off with one shot from his gun. In the ambulance, Davis explains to Fay her deranged boyfriend's motivation for killing. His hatred of sexually active women stemmed from bitterness at the cruel blow fate had dealt his young daughter, who will never enjoy the freedoms of his despised victims. After leaving the scene, the phone in the now-deserted house rings again. In her hospital bed, little Suzy is calling out for her father pleading to answer her call, as her voice is drowned beneath the everyday traffic of the city.