Retired jewel thief John "The Cat" Robie is suspected by the police in a string of burglaries on the French Riviera. When they come to his hilltop villa to question him, he slips their grasp and heads to a restaurant owned by his friend Bertani. The restaurant's staff are members of Robie's old gang, who have been paroled for their work in the French Resistance during World War II. They are angry at Robie because they are all under suspicion as long as the new Cat is active. When the police arrive at the restaurant looking for Robie, Danielle, the daughter of the restaurant's wine steward Foussard, spirits him to safety; she is a young woman who fancies him dearly.
Robie realizes he can prove his innocence by catching the new Cat in the act. He enlists the aid of an insurance man, H. H. Hughson, who reluctantly discloses a list of persons currently on the Riviera who own the most expensive jewelry. The American tourists Jessie Stevens, a wealthy nouveau-riche widow, and her daughter Frances top the list. Robie strikes up a friendship with them. Frances feigns modesty at first, but kisses Robie at the end of the night as she retires to her room.
The day after, Frances invites Robie to a swim at the beach, where Robie runs into Danielle. He keeps up his cover of being a wealthy American tourist, despite Danielle's jealous barbs about his interest in Frances. Frances accompanies Robie to a villa, where Robie suspects the new Cat might break in. Frances reveals that she knows Robie's real identity. He initially denies it, but concedes it that evening when she has invited him to her room to watch a fireworks display. They kiss passionately.
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 by the time they reach Jessie's room Robie has disappeared.
Later Robie, while staking out an estate at night trying to catch the thief, is attacked by an unknown assailant. A second attacker raises a wrench and tries to kill Robie but accidentally instead hits the first assailant, who falls off the estate's seawall into the water. When the police reach the body in the water it turns out to be Foussard.
The police chief announces to the press that Foussard was the jewel thief, but, as Robie points out privately in the chief's office with Hughson present, this would have been impossible because Foussard had a wooden leg and could not climb on rooftops.
Foussard's funeral is interrupted 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 asks Frances to arrange his attendance at a fancy masquerade ball, where he believes the Cat will strike again.
Robie accompanies Frances to the ball dressed as a masked Moor. The police hover nearby. When Jessie addresses the Moor as "John" and asks him to go and get her "heart pills", the authorities are tipped off 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 accompanies Frances to her room, he removes the mask and turns out to be Hughson, who switched places with Robie to conceal the latter's exit. Upstairs, the cat burglar silently cleans out several jewel boxes.
Robie lurks on the rooftop, and his patience is finally rewarded when he spots a figure in black whom he recognizes as Danielle, Foussard's daughter. The police throw a spotlight on him and demand that he halt, giving Danielle the chance to slip away. Robie flees as they shoot at him and manages to corner his foe with jewels in hand. She loses her footing on the roof and starts to fall, but Robie grabs her hand at the last second. While she hangs in his grasp, he forces her to confess to the police and admit that Bertani was behind the thefts.
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."
