In her family's Spanish villa, Anne Baxter as Kimberley Prescott, a young South African heiress of a diamond company, is grieving after her father's recent suicide. The mood is tense and claustrophobic as a man arrives claiming to be her brother Ward McKenzie Prescott Jr., Williams—Richard Todd in the role—who supposedly died in a car accident months earlier. He bears a driving licence, a passport, and a bank letter all in Ward's name, and even two upstairs photos bear a striking resemblance to him. The local police chief, Vargas, Herbert Lom, leaves the scene, convinced Kim has slipped into instability, and the house feels suddenly volatile and watched.
The following day, an unknown woman arrives claiming to be Mrs Whitman, a friend of Ward—Faith Brook in the part—adding another layer of doubt and danger to Kimberley's already fragile trust. Kim's maid has been sent away on extended leave, and a discreet butler is installed in the villa, as if someone is orchestrating a controlled environment around her. Kim reaches for help by trying to contact her Uncle Chan, who knows both the real Ward and her, but when Chan finally shows up, he greets the impostor as if he were the true Ward. This complicates Kim's suspicions, making her worry that the impostor is after her inheritance.
The plot thickens as Ward and Mrs Whitman press Kim to sign a will, while Ward himself raises a troubling accusation: he suggests Kim may have stolen diamonds from their late father's vault. He has a record of flights that reveals gaps in her alibi, fueling Kim's paranoia. Kim admits, under pressure, that she once took the diamonds to Tangiers, a revelation that seems to confirm the impostor's claim to some extent. The impostor and Mrs Whitman then arrange for Kim to sign an introduction that would position Ward as her agent to the bank in Tangiers, tightening their control over her finances and future.
Kim attempts a desperate escape to the beach house below the main villa. A follower corners her, and she nearly fires a spear gun in a confrontation, only to realize it is Vargas, who has come to intervene. He asks for something tangible—proof of Ward's fingerprints—since faking such evidence would be difficult. Kim manages to obtain a genuine fingerprint sample after meeting Ward again on the terrace, and the two share a tense moment of brandy-fueled flirtation while Mrs Whitman retreats upstairs. Kim races back to the beach house, retrieves a hidden metal box from the chimney, and tries to slip away through the front door, only to be blocked by Uncle Chan.
Her captors haul her back to the terrace and open the box, laying the diamonds out on the table as they demand she sign the will. Ward suggests they switch to a plan that includes a swim, while Mrs Whitman advocates taking a boat to safety. Kim, fearing they intend to drown her, bolts back into the villa. Vargas arrives again, and Kim pleads for rescue, hoping he will expose the impostor. He confirms that the fingerprints on the sample do match Ward's, which shakes Kim to her core. Overcome with a rush of confession, she blurts out that her brother must be dead—“because I killed him”—and admits that she had tampered with his car brakes to watch him crash off a cliff.
The twist lands with clinical precision: Ward and Mrs Whitman are undercover police, sent to uncover the missing diamonds and to uncover the truth behind the real Ward's death. The discovery reframes the entire nightmare Kim endured, turning a personal tragedy into a calculated investigation and revealing a coordinated effort that used her grief and fear to drive the deception. The finale ties together a web of family inheritance, staged identity, and a police operation that finally brings the truth to light, leaving Kim to confront the consequences of her actions and the haunting memory of a brother she believed she had lost—and perhaps never truly knew.
