On the private island retreat of the wealthy industrialist George Stark Teodoro Corrà, a weekend gathering brings together a mix of guests, including the renowned scientist Professor Gerry Farrell William Berger. The first night passes quietly, but the mood shifts when Farrell awakes to a startling realization: Stark and the others have set this weekend up to pressure him into selling his groundbreaking resin formula, a invention whose origin he guards closely after the death of a colleague during its creation.
Farrell’s world is tangled in personal conflict. His wife Trudy Ira von Fürstenberg is involved with Jill, Stark’s artist wife, while Stark’s business partner Nick Maurice Poli is openly abusive toward his wife Marie Edwige Fenech, yet shows little objection to her sleeping with other men, including Charles Mauro Bosco, the Stark household’s manservant. Stark himself treats Jill more as a means of enhancing his portfolio than as a partner, constantly steering her career and keeping tight control over every aspect of her life. The only truly contented couple appears to be Jack Howard Ross and Peggy Helena Ronee. Also present is Isabel Ely Galleani, the teenage daughter of Stark’s game warden, whose parents are away for medical reasons.
As the trio of Stark, Nick, and Jack press Farrell for the secret formula, they each try to seal the deal with conspicuously large cheques—$1 million apiece—from their Swiss accounts. The pressure mounts in the open air and the tension soon turns grim. Jill discovers the dead body of Charles on the beach, a grisly clue that someone on the island might be willing to go far to protect the prize they crave. With the motor launch already sent away to block any escape and the radio out of service, Charles’s body is moved into a large walk‑in freezer, a place that will later become a macabre locus for the day’s horrors. The next morning, Farrell is found on the beach, and the group learns in horror that he has been murdered; Isabel, the sharp-eyed sniper, drags Farrell’s body toward the sea as the situation spirals.
Tempers flare as the island devolves into a chain of killings. Peggy is shot dead from an unseen sniper, and Jack immediately accuses Stark of the killings. Marie is found bound to a tree and stabbed in the chest, while Jill is discovered dead in her bathtub, wrists slashed in what appears to be a staged suicide. Each corpse is disposed of in the freezer, turning the island into a chilling gallery of cadavers. The four survivors—Stark, Jack, Nick, and Trudy—hole up in Stark’s living room through the night, trading barbs and accusations as the danger closes in. After a heated exchange, Nick storms off, and the next morning he too is found dead, his body added to the growing cold-storage tableau.
Stark tries to salvage some chance of escape by offering Trudy a cheque for Farrell’s formula, while a microfilm reveals that the secret remains in existence. A hidden motorboat is unearthed, offering a path back to the mainland, but Stark’s attempt to gather supplies takes him away, leaving Jack to confront the truth: he orchestrated the murders to steal the cheques, including Peggy’s, and has now turned to lethal means to protect his plan. In a tense confrontation, Trudy agrees to deliver the formula in exchange for Jack’s cheque, and a dangerous handover ensues. Yet the exchange ends in tragedy as they both shoot each other; Trudy dies first, and Isabel seizes both the cheque and the formula, though a dying Jack makes a last, futile attempt to stop her.
Time passes, and Isabel—now wealthier than before—visits Farrell in prison, awaiting execution for the murder of his colleague, the rightful discoverer of the resin. She claims to love Farrell and confesses that she intended to save him by drugging him with a sodium pentathol pellet, a truth-serum style move that coerces him into confessing his guilt. Isabel reveals she is broke, having cashed George’s and Jack’s cheques but not Nick’s because she never learned the account number. Farrell, aware of his fate, provides her with the number before she departs for Lausanne, convinced she has altered the course of history on that distant island.
