On a small tropical island, wealthy, beautiful white islander Venus de Viasa is being unwittingly used by the natives to assist in voodoo rituals led by their powerful priestess, Mama Rataloy. Coincidentally, Venus begins dating a successful visiting author, J. Peter Weils, who is conducting research for a study on voodoo that he intends to expose as a dangerous sham based on ignorant superstitions. While frolicking on the beach one afternoon, Peter and Venus find the body of a dead man with a long hat pin piercing his side. Frightened, Venus acknowledges that the pin is a sign of voodoo and pleads with Peter not to report the death to the police. Back at Venus’ home, Peter tells her that he has examined numerous police reports on several mysterious, unsolved deaths that he believes were part of the island’s regular nightly voodoo ceremonies. Despite Venus’ warning not to get involved with the island’s voodoo traditions, Peter makes plans to visit Mama Rataloy. Later that day, Peter welcomes his daughter Sarah and her new husband Warren, who have arrived to spend their honeymoon on the island. At a club that evening, Sarah is disturbed by Venus’ beauty and frank flirtation with Warren, but Peter reassures her that Venus means no harm. The following day, a guide, arranged by Peter’s cook Symanthemum, takes the writer to the beachside cabin of Mama Rataloy. The priestess reveals she is aware of Peter’s inquiries and advises him not to tamper with things of which he has no knowledge. Hearing a noise in another room, Peter investigates and is stunned to find a sick and weak native man tied against the wall. Mama Rataloy explains the man is receiving just punishment for being a disbeliever in the order of the snake. Peter presents Mama Rataloy with the hat pin, but when she threatens him, Peter assures her that he is not afraid of her powers. After Peter’s departure, Mama Rataloy takes another poisoned hat pin and gouges the imprisoned man in the eye. A few days later, Peter receives a skull with a hat pin impaled in the eye socket. When Sarah and Warren see the skull and express shock, Peter explains the skull is a “fix” or a warning. Despite his daughter’s concern, Peter says he is more determined than ever to publish a book to educate people by exposing the dangerous mystery of voodoo practices. The next day, Sarah takes dictation from her father on his book as he describes the voodoo belief in the “Dombella,” or the serpent of revenge that destroys unbelievers every decade. As Peter describes the appearance of the Dombella, a serpent that enters the body of a woman and transforms her, Sarah anxiously notes that the serpent goddess is due that year. Peter dismisses his daughter’s anxiety and insists he will link Mama Rataloy’s actions with the numerous unsolved murders on the island. Later, Peter accompanies Warren and Sarah down to the beach. While the couple swims, Peter remains on the beach, unaware that Mama Rataloy lurks nearby. A barking dog attracts Peter to the body of a man who is menaced by a snake. After Peter frightens off the snake with a stick, Warren and Sarah join Peter to examine the man, and discover he is dead. Peter contacts police inspector Escoberto who thanks him for his help, but insists that he take caution when dealing with Mama Rataloy. Upon returning home, Peter is surprised to find the priestess waiting. She ominously cautions him that the time of the serpent is at hand, then departs. That evening, Warren takes Sarah to dinner and while the couple embraces on an outside terrace, Sarah spots a strange woman robed in white lurking in the bushes. The unnerved couple contact Escoberto and Sarah insists the woman’s head resembled that of a snake. Meanwhile, Peter calls on Venus at her home. Confessing that she has been suffering recent memory lapses, Venus then reveals the lapses and sensation of not being herself has made her fear for her sanity. Peter asks if she has had contact with Mama Rataloy and Venus shows him a ring given to her by the priestess for protection. Peter notes the ring has made a small scratch on Venus’ hand and suspects that she is being drugged. Insisting she take a sedative and retire, Peter departs in search of Mama Rataloy. In a trance, Venus drinks a potion from a small vial. Back at Peter’s home, Warren is unable to locate Symanthemum and while looking for her outside is knocked out and carried away. Peter is soon attracted by the sounds of drums and follows them through the dark as a native ceremony begins. Mama Rataloy leads the ritual with a dazed Venus wearing a white robe at her side. When Peter arrives and tries to hide in the trees, he is abruptly grabbed and held by two men. To his horror, Peter sees that an unconscious Warren is the sacrificial victim tethered to stakes in the ground. As the ceremony becomes more and more frenzied, Mama Rataloy presents Venus with a snake skin holding a poison hat pin, which Venus takes and then goes to stand over Warren. As Venus is about to stab Warren with the pin, Escoberto, Sarah and several police officers arrive, frightening the natives, who flee. Mama Rataloy collapses and Venus faints as Sarah frees Warren. While the police detective and Peter watch, the voodoo priestess turns into a snake, then bursts into flames and disintegrates. Venus awakens with no memory of the events and although Peter is relieved his family is safe, Escoberto observes that voodoo remains a very real mystery.