At the private island retreat of wealthy industrialist George Stark, a group of people, including scientist Gerry Farrell, have assembled for a weekend getaway. After the first night, Farrell is enraged the next morning to discover that Stark and the other guests planned the getaway to persuade him to sell his latest invention, a formula for a revolutionary industrial
resin, which he is reluctant to divulge due to his colleague's death during its invention. Farrell's wife, Trudy, is having an affair with Stark's artist wife, Jill. Stark's business partner, Nick, is verbally abusive to his coquettish wife Marie but does not object to her
sleeping with other men, one of whom is the Starks' manservant, Charles. Stark is less of a husband to Jill than a business manager; he arranges for her paintings and artwork to be publicly displayed but is a source of constant criticism. The sole happy couple is Nick's co-worker Jack and his wife Peggy. Also on the island is Isabel, the teenage daughter of Stark's
game warden; Isabel's parents are away for medical reasons. As Stark, Nick and Jack badger Farrell for the formula—the original documents for which he has secretly destroyed—by offering him cheques for $1 million each from their
Swiss bank accounts, Jill discovers the dead body of Charles on the beach. Having already sent the motor launch away to prevent Farrell from leaving the island, and with the radio out of commission, Stark has no way to contact the mainland. Charles's body is moved into a large walk-in freezer. The next morning, Farrell walks alone on the beach. Trudy and Jill, walking hand-in-hand nearby, hear a gunshot and find Farrell's body. They run away to tell the others. The sniper, Isabel, drags Farrell's body to the sea. As tempers flare after Farrell's apparent death and disappearance, more killings occur. Peggy, standing on the balcony of her room, is shot dead by an unseen assailant. Jack arrives on the scene first and accuses Stark of being responsible. Marie is found to have been tied to a tree and stabbed in the chest. Jill turns up dead in her bathtub, her wrists slashed in an apparent suicide. Each of the bodies is placed in the freezer. The four remaining survivors—Stark, Jack, Nick, and Trudy—hole up in Stark's living room for the night. After bickering with Trudy, Nick storms off. The next morning, he too is found dead, and his body is placed in cold storage. Stark offers Trudy his cheque for Farrell's formula, which she reveals still exists on a microfilm, and uncovers a previously hidden motorboat which can take them to the mainland. As Stark returns to the house to get supplies, Jack confronts Stark and reveals he killed everyone except for Farrell to steal their cheques and eliminate any witnesses. Jack shoots Stark dead. Trudy offers Jack the microfilm for his cheque. After the exchange, they both shoot each other. Isabel steals the cheque and the formula, but not before a dying Jack tries to stop her. Some time later, a now-wealthy Isabel visits Farrell, alive and in prison, where he awaits execution for the murder of his colleague, the rightful discoverer of the resin. Isabel claims that she loves Farrell and had decided to save him by rendering him unconscious with a
sodium pentathol pellet of the kind her father had used to tranquilize animals, but was unaware of sodium pentathol's
truth serum properties, leading him to confess to killing his partner to the authorities. Isabel reveals that she is broke, having cashed and spent George's and Jack's cheques but not Nick's due to her not knowing the account number. Aware of his inescapable fate and thankful to her, Farrell gives Isabel the number, and she happily leaves, instructing her chauffeur to drive her to
Lausanne. == Cast ==