Multimillionaire Ernesto De Cardi Joseph Lenzi and Sandra Latham Janet Leigh, his new American bride, arrive at a spacious villa in Batres, Spain, where they are welcomed by an elderly servant, Juanito Juan Elices. Sandra is awestruck by the villa’s scale, taking in the sweeping views and the heavy sense of wealth that surrounds them. Yet the moment is punctuated by the din of loud blasting nearby, a project to widen a mountain road that keeps intruding on their private thoughts. Sandra longs for solitude with Ernesto, and he agrees to let Juanito go back to his farm so they can steal away into the villa’s vast rooms and corridors. After a night of heavy drinking, they retreat to bed, and the Rolls-Royce that carried them away from the outside world drives off in the middle of the night, leaving only the memory of the party behind.
When Sandra wakes, Ernesto is nowhere to be found. Days pass with no sign of him, so she goes to the small police department, where the sergeant calls Captain Sevilla [Rossano Brazzi]. Sevilla then contacts Ernesto’s attorney, Frederico Caprio [Eric Braeden], who explains that Ernesto has a long history of pursuing women and then leaving them after a short period, though Sandra is the first he has married. Frederico also promises to bring Carla [Barbara Steele], Ernesto’s sister, into the loop, confident she can help locate Ernesto. A startling turn arrives when someone arrives at the villa claiming to be Ernesto, and Sandra insists that this man is an impostor. He argues that the marriage was impulsive and that he regrets it, and after a tense confrontation, he is invited to dinner with Sevilla. The attorney hints that a settlement for an annulment will be offered the next day, and the wheels of a quiet battle begin to turn. Meanwhile, the sergeant suggests arming the police car with a siren and rotating light to bolster authority, a proposal Sevilla half-resists but ultimately accepts.
Frederico makes a monetary offer—$25,000 in exchange for an annulment—and Sandra refuses. She remembers the old servant, Juanito, and hopes to use him to identify Ernesto as an impostor, but when Sevilla brings the man in, the servant greets the purported Ernesto warmly, as if the two sides are simply acquaintances in a familiar home. Sevilla explains that Carla will be arriving soon, and she can provide confirmation that might clear up the matter. The trio move to a café, where Carla also greets Ernesto as if he were genuine, a moment that makes Sandra doubt her own instincts and suspect a larger deception at work. Back at the villa, a new settlement is proposed, adding the villa itself to the cash offer, but Sandra refuses and Frederico threatens to have her committed if she continues to resist. Then the truth begins to spill out: the fake Ernesto reveals that the real Ernesto once injured a woman badly, and he, Antonio, is a stand-in who has been recruited to keep people from revenge.
Carla decides that she wants to keep the villa, and the settlement is withdrawn. Sevilla, strolling through the night streets, spots Ernesto’s Rolls-Royce and recognizes the Spirit of Ecstasy hood ornament—an object that the sergeant had previously found and turned in from a demolished site. The trio offer Sandra a new agreement, but she remains defiant and tears up the contract. Antonio and the others realize that Sandra’s knowledge and timing could unravel everything, and when she mocks their plan, she bursts into laughter, admitting that she studied Ernesto for a year, learned how he left women, and used his hunger to her own advantage. She had drugged Ernesto’s champagne the night he disappeared, placed him in the Rolls-Royce, driven him toward a mountain road, and pushed the car over the edge, intending for it to explode in the gully so Ernesto’s death would look like a drunken accident. However, the accident sank into the dirt and was buried beneath newly poured demolition debris from the road project. She did not know about Antonio, but she used the trio’s masquerade to make herself more believable, and she recognized that their scheme could inadvertently incriminate her if they pursued reporting the death. All the while, she believed she had an ally in Sevilla, who had begun to fall for her.
Sandra demands a million dollars. Sevilla hurries back to the villa and declares that the case is closed—until he notices a clue that suggests there were two different Rolls-Royce cars, implying two Ernestos. He resolves to begin a search at the demolition site the next day to locate the real Ernesto’s body. Afraid of having her fragile plan exposed, Sandra drugs Sevilla again, hoping to silence him, and when he blacks out she drags him into his police car and drives toward the cliff. A wheel catches on a rock, and as she struggles to shift the rock, the awake Sevilla clocks the new siren and the town sergeant follows the sound. Sevilla wakes, confronts Sandra at the road’s edge, and she slides into a nervous breakdown. In a tense, restrained moment, Sevilla persuades her to step back from the precipice and then drives her to town in custody, bringing an ending that stays true to the intentions of the night’s dangerous masquerade.
