A retired cat burglar fights to clear himself of a series of Riviera robberies committed in his style.

After a series of daring, nighttime jewel thefts creates panic among the Riviera's wealthy elite, American-born John Robie, a reformed burglar who used to be known as "The Cat," becomes the police's only suspect. When police detectives come to question him at his hilltop villa near Cannes, John cleverly eludes them. John then races to see his old friend, Bertani, a restaurateur with whom he fought in the French Resistance and whose employees are all ex-convicts like John. Although Bertani is sympathetic to John's plight, the other restaurant workers treat him hostily, fearing that his apparent transgression will cast suspicion on them. Feeling that his only recourse is to catch the thief himself, John asks Bertani for information about his rich customers. Instead, Bertani offers to put John in contact with a man who two days before asked for the same information. As John is leaving the restaurant, he is spotted by the still-pursuing police detectives. Foussard, one of Bertani's workers, helps John flee and delivers him to his teenage daughter Danielle, who takes John out in a motor boat. The flirtatious Danielle then drops John off at the Carlton Hotel beach, where unknown to him, he is observed by a beautiful woman. At the hotel, John receives a phone call from Bertani, instructing him to meet the man, H. H. Hughson, at the flower market in Nice. There, John explains his situation to Hughson, an English insurance investigator from Lloyds of London, a company that has been hard hit by the recent thefts. Though a bit wary, Hughson agrees to supply John with a list of the thief's probable targets. John then is chased by detectives through the flower market, but again avoids capture. The next day, at John's villa, Hughson hands John his list of clients and admits he is dining with two of them, American tourists Mrs. Jessie Stevens and her grown daughter Frances, who are staying at the Carlton. Posing as an Oregon lumber magnate named Conrad Burns, John connives to meet Jessie and Francie, the woman from the beach, while they are gambling in a casino. Although he shows no particular interest in Francie, the widowed Jessie, impressed by his manliness, encourages him to pursue her spoiled, refined daughter. At first, Francie acts indifferent to John, then surprises him with a seductive goodnight kiss. Despite increased security at the hotel, another jewel theft occurs that night. The next morning, John, who has checked into the hotel, receives a note warning him that his life is in danger, and as he is sunbathing on the beach with Francie, Danielle walks by. John follows her into the water, and they swim out to a platform, where Danielle informs John that Bertani's ex-convicts have been threatening to kill him. Later, Francie drives John to inspect one of the villas on Hughson's list, which he claims he is interested in renting. While inspecting the layout of the expansive Silvers villa, John accuses Francie of being insecure about men. Unfazed by John's criticisms, Francie insists that they picnic together, but while driving, John realizes they are being followed by the detectives. Stepping on the gas of her sporty convertible, Francie engages the policemen in a wild chase on the steep, winding Riviera roads and finally loses them. Later, as they picnic, Francie reveals that she knows John's real identity and begs him to make her his accomplice. John maintains his pose, however, but agrees to meet her later in her hotel room. Francie, who normally shuns jewelry, greets John wearing an extravagant diamond necklace and tries to taunt him with it. After John points out that the gems are fake, he and Francie kiss, growing increasingly passionate as a fireworks display lights up the night sky. The next day, however, Francie storms into John's room, demanding he return her mother's jewelry, which was stolen during the night. Although John admits he is The Cat, he protests his innocence and slips out to the roof when Francie summons the police. John, who has received yet another warning note, then meets with Hughson and asks him to bring the police to the Silvers villa after midnight, as he expects the burglar to be there. While waiting on the villa grounds, John is grabbed by two attackers. In the ensuing struggle, John causes one of the attackers to plunge off a bridge into shallow water. The police identify the dead man, Foussard, as The Cat and clear John of suspicion. John, however, tells the police that Foussard, who had a peg leg, could not be the burglar and arranges with an apologetic Francie to attend an upcoming costume ball at the Sanford estate. Wearing an exotic costume complete with black mask, John accompanies the Stevenses, who are dressed in Louis XIV-era gowns. During the ball, which is being monitored by costumed policemen and catered by Bertani, John excuses himself, then apparently returns and dances with Francie late into the night. Hughson actually has replaced John, who is waiting on the roof for The Cat. Finally, John spots the burglar and gives chase. John soon nabs the masked thief, who turns out to be Danielle. While trying to escape John's grasp, Danielle slips off the roof, but catches the gutter before falling. John grabs her hand and threatens to let go unless she tells the police down below that she and her father were working with Bertani. After Danielle confesses and is rescued, Francie finds John at his villa and forces him to admit with a kiss that he needs her after all. The triumphant Francie then states that "Mother will love it here."