The pattern of a string of jewel robberies on the French Riviera causes the police to believe that the infamous jewel thief John "The Cat" Robie (Cary Grant) has left his comfortable retirement of growing grapes and flowers. He gives the police the slip at his hilltop villa. Robie visits a restaurant. The staff is his old gang, paroled for their work in the French Resistance during World War II as long as they keep clean. Bertani, Foussard, and the others angrily blame Robie because they are all under suspicion so long as the new Cat is active. When the police arrive at Bertani's restaurant looking for Robie, Foussard's teenage daughter Danielle (Brigitte Auber), who has a crush on him, spirits him to safety.
Robie recognizes he can prove his innocence by catching the new Cat in the act. He enlists the aid of an insurance man, H. H. Hughson (John Williams), who reluctantly discloses a list of the most expensive jewelry owners currently on the Riviera. Tourists Jessie Stevens (Jessie Royce Landis), a wealthy nouveau riche widow, and her daughter Frances (Grace Kelly), top the list. Robie strikes up a friendship with them. Jessie is delighted, but Frances offers a pretense of modesty. When Robie and Frances run into Danielle at the beach, Robie keeps up the mask of being a wealthy American tourist, despite Danielle's jealous barbs about his interest in Frances.
Frances sees through Robie's cover. She seduces him, dangling before him in her jewels. She teases him with steamy tales of rooftop escapades and offers herself as an accomplice who might share his crimes. Fireworks fill the night sky.
The next morning, Jessie discovers her jewels are gone. Frances accuses Robie of using her as a distraction so he could steal her mother's jewelry. The police are called, but Robie has disappeared.
To catch the new Cat, Robie stakes out an estate at night. He struggles with an attacker. A second attacker raises a wrench and appears to hit Robie, who falls off the estate's seawall into the water. But it is Foussard who dies in the fall. The police chief publicly announces that Foussard was the jewel thief, but, as Robie points out privately in the abashed Hughson's presence, this would have been impossible because Foussard had a wooden leg, and could not climb on rooftops.
Foussard's funeral is marred by Danielle's loud accusation that Robie is responsible for her father's death. Outside the graveyard, Frances apologizes to Robie and confesses her love. Robie needs to continue his search for the Cat. He asks Frances to arrange his attendance at a fancy masquerade ball, where he believes the Cat will strike again.
At the ball, Frances is resplendent in a gold gown, whilst Robie is unrecognizable behind the mask of a Moor. The police hover nearby. Upstairs, the cat burglar silently cleans out several jewel boxes. When Jessie asks the Moor to go and get her "heart pills," Robie's voice tips off the authorities as to his identity. Upon the masked Moor's return, the police wait as he and Frances dance together all night. When the masked Moor and Frances go to her room, the mask is removed: it was Hughson, who switched places with Robie to conceal Robie's exit.
Robie lurks on the rooftop, and his patience is finally rewarded when he spots a figure in black. However, just as his pursuit begins, the police throw a spotlight on him and demand he halt. He flees as they shoot at him, but he nonetheless manages to corner his foe with jewels in hand. Unmasked, his nemesis turns out to be Danielle. She loses her footing on the roof, but Robie grabs her hand before she can fall. He forces her to confess the father-daughter involvement loudly to the police and that Bertani was the ringleader of this gang.
Robie speeds back to his villa. Frances follows to convince him that she has a place in his life. He agrees but looks less than thrilled when she says, "Mother will love it up here."
