Suave and debonair John Robie is a reformed jewel thief. He was once the infamous burglar known as The Cat who was a wanted man across practically all of Europe. Noted for his daring nighttime pinches, he has become the prime suspect after a rash of thefts leave many of the Riviera’s most elite weighing slightly lighter during formal occasions.
The police arrive to interrogate Robie at his villa in the South of France, but Robie easily eludes them as he heads straight for Bertani. Bertani is a comrade from their days in the French Resistance who now own a restaurant that employs only ex-cons. Bertani turns out to be welcoming enough, but his employees eye Robie with suspicion and hostility. After all, when you are known by the company you keep, the last company you want to be keeping with is a man every police officer in the area is looking for. Robie make inquiries about Bertani’s wealthiest customers, but instead gets the name of a man who showed up two days earlier asking for the very same information.
The police having track him down, Robie is forced to flee with the assistance of Foussard, one of Bertani’s cons, and Foussard's daughter, Danielle. Eventually, he reaches the Carlton Hotel where he falls under the eye of a beautiful young blonde. Inside the hotel he receives a phone call from Bertani setting up the details of a meeting with H.H. Hughson. Hughson is an insurance investigator for Lloyds of London that has recently taken a pummeling from claims made by victims of jewel theft. Suspicious of his intentions, Hughson nonetheless pass along a list of potential targets. The police bear down again and a chase ensues with Robie once again proving elusive.
Among the names on the list are Mrs. Jessie Stevens and her daughter Frances who just so happen to be guests at the Carlton. Robie meets them in the guise of a lumber baron named Conrad Burns and Jessie is impressed enough to encourage the development of a relationship between him and her daughter. After a surprisingly aggressive kiss from the standoffish Frances, another theft takes place and the next day Robie is informed that his life is in danger.
While enjoying a picnic, Frances surprises Robie again by informing him that she knows his real identity. The follows a third surprise: she wants to become his partner in crime. That night she shows up in his room sporting an enormous diamond necklace with which she mercilessly teases him until he lets her know that he knows it’s a fake. They start to make love.
The next morning Frances crashes into his hotel room with news that her mother’s jewels were stolen overnight. Then she demands he return them. Robie fully admits to being The Cat, but swears he didn’t steal from her. Frances refuses to believe him and starts dialing the police, which sends Robie on the run again.
After another warning that his life is in danger, he meets up with Hughson and requests to meet with the police at his villa after midnight. He is also expecting the real burglar. As he waits, two men assault him but in the struggle one falls over the bridge into the water. The man is identified as Foussard and Foussard is identified as The Cat. Suspicion falls off Robie. Robie confesses that Foussard had a peg leg which makes it impossible for him to have been The Cat.
Frances come to apologize and plans are made for them both to attend a costume ball at the estate of one of the elite. Robie shows up in a costume topped by a black mask and Jessie and Frances are attired in pre-French revolution gowns. The ball is being watched closed by police also in costume and is catered by Bertani’s restaurant. Robie excuses him but quickly returns to dance the night away with Frances. In fact, it is Hughson with Frances, dressed in the same costume that Robie has been wearing. Robie is now on the roof, awaiting the imminent arrival of the person who has taken over as The Cat.
A chase takes place after the thief is spotted by Robie, but eventually overtaken and unmasked. The Cat turns out to be none other than Foussard’s daughter, Danielle. She struggles to escape Robie’s clutches, but slips and nearly falls off the roof, grabbing hold of the gutters at the last second. Robie takes her hand, but blackmails her into telling the police the whole story about her working in tandem with her dad and Bertani. If she doesn’t agree, he’ll let go. She agrees, confessed and is rescued.
Frances appears at Robie’s villa and manages to convince that she needs him and he needs her. The happy ending is only interrupted for Robie by the final surprise Frances delivers to Robie: how much her mother will enjoy living with them at his villa.