At a remote mansion overlooking a vast, empty bay lake, Countess Federica (Isa Miranda), wheels herself through her dark, empty house. She sorrowfully looks out her window at the rain when Filippo Donati (Giovanni Nuvoletti), her husband, appears and murders her by wrapping a noose around her neck and kicks her out of her wheelchair. The countess cannot stand on or move her useless legs and is strangled to death. But then Filippo is then murdered by a unseen killer who jumps out at him from behind a curtain, and drags his body away. The presence of a suicide note, stolen from the countess' diary, leads the police to believe that she has killed herself.
Several days later, Frank Ventura (Chris Avram), a real estate agent, and his secretary/lover Laura (Anna Maria Rosati) are in bed discussing their plans to take over Countess Federicas property following her murder. They both realized the bay's potential to fetch a large price on the estate market, and they are the ones who have convinced Donati to murder his wife. All that Ventura needs to take control of the bay is Donati's signature on some paperwork, although he does not realize that Donati too has been murdered. Ventura plans to go to his cottage, which adjoins the desired property, to meet with Donati. Laura, jealous of Ventura's obsession with the property, asks to come along. When he tells her to stay put, she complains that he enjoys making her life miserable. Ventura tells Laura about a 'squonk' which is mystical creature who is crafty, but never stops whimpering. Laura understands the analogy and agrees to be patient.
Back at the bay, Paolo Foscari (Leopoldo Trieste), an entomologist who owns a small house on the grounds, is hard at work trying to capture an insect. He runs into Simon (Claudio Volonte), a handyman who also lives on the property and makes a living by catching squid in the bay. During their conversation, Foscari suggests that the countess might have been murdered. Very defensively, Simon replies that the police ruled it a suicide. Foscari sarcastically replies that it was a suicide before running off to catch more insects.
Meanwhile, four rowdy teenagers, Brunhilda (Brigette Skay), Denise (Paola Rubens), Duke (Guido Boccaccini), and Robert (Roberto Bonanni), arrive at the bay for a day of partying and fun. The trouble-making teens break into Ventura's vacant cottage to steal some liquor and smoke some dope. While Denise and Duke retreat to a nearby bedroom for sex, Robert, makes a fire in the fireplace, while Brunhilda goes swimming in the bay. There, she accidentally discovers the badly decomposed body of Filippo Donati floating under water. In a panic state, she runs out of the water, quickly dresses, and runs back to the cottage to tell the others of her find, only to get chased and attacked by a unseen maniac wielding a large billhook, who catches up to her and slashes her throat. Robert is next when he opens the front door of the cottage after hearing a noise and is whacked in the face with the same billhook. The unseen killer takes a fisherman's spear, and kills both Denise and Duke by impaling them in bed while they are in the middle of sex.
As night falls on the bay, Renata (Claudine Auger), Donati's estranged daughter, arrives in a mobile home with her weak-willed husband Albert (Luigi Pistilli), hoping to lay claim to the bay estate. Leaving their two young children asleep in their camper, the couple pay a visit to Foscari and his wife Anna (Laura Betti), a fortune teller and tarot reader with a very supercilious nature. Sitting down for tea, Anna slyly insinuates that the countess' death was probably Renata's father's doing, and that Simon may be the one to inherit the property. Shocked to discover that she has a half-brother, Renata leaves with Albert who tells him that Simon will have to be disposed of.
Renata and Albert meet with Simon to talk with him about the countess' death when they discover the badly decomposed body of Donati's in Simon's boat. Simon claims to have only found the body floating in the bay and will inform the authorities. Renata and Albert flee to stop at Venturas cottage hoping to meet with him, but find it apparently deserted. When Albert walks away for a moment, Ventura suddenly appears and attempts to kill Renata. She runs into the bathroom where she discovers the four mutilated, dead bodies of the teenagers. But Renata gets the upper hand by grabbing a pair of scissors, and stabbing Ventura. This is accidently witnessed by Foscari, and when he runs back to his house to call the police, Albert follows him there and strangles him to death. When Anna, hearing the commotion at Ventura's cottage, arrives there and gets decapitated by Renata weilding an rafting axe. Frightened by the aura of violence around him and disgusted with himself for killing an innocent man, Albert tries to persuade Renata to make a run for it. But the murderous Renata tells Albert that she has no intention of giving up when she's so close to inheriting the bay. She also reminds Albert that they still have to kill Simon.
Things gets more complicated when Laura shows up at the bay looking for Ventura, when she stumbles into Simon's shack on the bay where he appears and threatens her with a machete. It is revealed by Simon that Ventura hired him to murder Filippo Donati with the promise that Ventura will give him his share of the property and money to leave the country. Simon really is the countess illegitimate son who was supported by his mother all this time and the one who murdered the four teenager after one of them discovered the body of Donati since he was ordered by Ventura not to let Donati's death be made public knowledge until after Simon inherits the bay. Furious at Laura when he finds out that she and Ventura were responsible for Donati murdering his mother, he attempt to kill her too. But at first Laura gets the upper hand by throwing a pot of boiling water on Simon, scalding his face. But he quickly recovers and strangles her with his bare hands.
Simon then goes after both Renata and Albert hiding out in the property. But then Albert turns the tables on Simon by impaling him to a wall with the same fisherman's spear Simon used to kill two of the teens. With Simon finally taken care of, Renata and Albert search the countess' house for the missing will which has the listings of beneficiaries. But then Ventura, who was presumed to be dead, shows up and attempts to kill both of them, but is then dealt with by Albert who kills him for good.
The next morning, with all the loose ends tied up and the countess will burned, Renata and Albert plan to return home to wait for news of her inheritance of the bay. But in an ironic twist, both Renata and Albert are shot to death by the very own neglected children, who playfully point a loaded shotgun at their parents and shoot them both down. The little girl says to her brother, "Gee, they sure are good at playing dead, aren't they?" The sister and brother then run out of the camper to playfully skip some stones by the water.