Plot

A rich, alcoholic widow assuages her loneliness through a ménage à trois with two seductive step-siblings, whose true motives threaten her sanity and life.

Kathryn, a glamorous American widow, arrives in Italy several weeks after the death of her older, extremely wealthy husband. With the help of Brian, her lawyer, she moves into a luxurious villa and proceeds to lead a lonely, uneventful existence until one day, a handsome young man, Peter shows up at the front gate, looking for tools so he can fix his sports car. She lets him stay the night, and the next thing she knows, she's madly making love to him in the shower. He eventually moves in, and is soon joined by a free spirit he introduces as Eva, his sister. She enjoys their company and partying with them - until she begins to suspect that they aren't what they seem to be.—Eugene Kim <genekim@concentric.net>

The wealthy American widow Kathryn travels from New York to Italy to rest in an isolated mansion nearby Rome. Her lawyer Brian makes the arrangements to rent the manor, with the maid Teresa "Terry" and the housekeeper and gardener Martino. Kathryn is bored and drinks whiskey and uses pills during all the day. When she meets the younger Peter, who asks to use her telephone to call the mechanic to repair his car, they have a love affair and Kathryn has a crush on him. She visits him at his apartment, but he is evicted by his landlord. Kathryn invites Peter to stay in the house as her guest. Soon Peter's sister Eva appears in the house, and Kathryn invites her to stay in the house. Peter and Eva convince Kathryn to send Teresa on vacation and soon they have a threesome and Kathryn learns that Eva is Peter's mistress. She sends them away, but they return with photos to blackmail her. Kathryn is drugged by them and soon she learns their true intention.—Claudio Carvalho, Rio de Janeiro, Brazil

American socialite Kathryn West (Carroll Baker) arrives in Italy from New York to a flurry of journalists following the car accident death of her husband Robert, a Texas oil baron who left her his $200 million estate. She retreats to an Italian villa rented by her austere lawyer, Brian Sanders (Tino Carraro). Kathryn soon meets Peter Donovan (Lou Castel), a young American man from Boston whose car has broken down nearby. Peter manages to manipulate his way into staying with Kathryn, who is initially icy toward his romantic advances. However, Peter swiftly manages to seduce her, and the two engage in a passionate affair.

One day, Kathryn travels to London, England to visit with her late husband's relatives to discuss property she is bequeathing to them, but finds them to be abrasive and angry with her, believing she married Robert solely for his money. Late one night, Kathryn senses someone has broken into the house, but the housekeeper Teresa (Lilla Brignone) attributes Kathryn's paranoia to her abuse of alcohol and prescription drugs. When Kathryn invites Peter to stay with her for a week, his sister Eva (Colette Descombes) arrives at the villa unexpectedly. Kathryn enjoys the siblings' companionship and youthful vivaciousness.

During a dinner meeting with Brian, Kathryn admits she is suffering from liver failure due to her alcoholism. Upon returning to the villa, Kathryn finds Peter and Eva in bed together nude. She presumes the two have engaged in incest, but Peter informs Kathryn that he and Eva are not blood-related, and are in fact only step-siblings. Eva confesses that she is sexually attracted to Kathryn, and the three of them soon engage in a series of nightly debauchery threesomes fueled by Peter and Eva's indulging of Kathryn in liquor and stimulant drugs.

One night, Kathryn, disgusted with herself and in a drunken rage, orders Peter and Eva to leave the villa. They oblige, but soon return, and begin physically abusing Kathryn and taking control of the house, subduing Kathryn by forcing her to drink. Finding herself held hostage by the siblings, Kathryn becomes increasingly desperate to escape Peter and Eva. She manages to produce a gun, with which she shoots Peter. Kathryn loses consciousness due to a sedative fed to her by Eva, and awakens to find that the gun was loaded with blanks by the siblings, and that Peter was unharmed. The two bind and gag Kathryn, tying her to a bed. When Brian visits the villa for a visit, Peter tells him that Kathryn has descended into a depression, and has threatened to commit suicide.

Locked in her upstairs bedroom, Kathryn is further tormented by the siblings, who on one occasion serve her a live toad for dinner. The two later bring her whiskey and a large number of barbiturates, along with a falsified letter stating Brian has died in a plane crash, with the hope that Kathryn will voluntarily kill herself by overdosing. Kathryn manages to escape her bedroom and flees to the attic, accessing the roof of the villa. Brian arrives, and witnesses Kathryn collapse over the edge of the roof, falling to the veranda below and fracturing her skull. Brian picks the injured Kathryn up in his arms. The two briefly make eye contact before he ruthlessly throws her over the veranda onto the driveway below, killing her.

Brian, who has conspired with the siblings to steal Kathryn's fortune, produces two forged suicide notes, and orders Peter to alert authorities to Kathryn's staged suicide. It is revealed that Brian is in fact Peter and Eva's uncle, and had Kathryn unknowingly sign documents that bequeathed her estate to the trio.

After the reading of Kathryn's last will and testament, Brian is met by a detective who notifies him that New York police discovered that the brakes in Robert's car were tampered with, causing his fatal road accident. Brian momentarily fears their plot has been discovered, until the detective states that police believe Kathryn caused the accident, as Robert had planned to divorce her. Based on this conclusion, Kathryn is not legally entitled to her husband's estate, which Brian is informed will instead go to his aunt... thus by fate Brian has inherited nothing of Kathryn's money. Meanwhile, as Peter and Eva gleefully drive through the city in their convertible, they are killed in a head-on collision with a truck.