To Catch a Thief

John Robie (Cary Grant) is a retired infamous jewel thief or "cat burglar", nicknamed "The Cat", who now lives in a hilltop villa in the French Riviera growing grapes and flowers. The modus operandi of a recent series of robberies leads the police to believe that Robie is active again; they attempt to arrest him, but he gives them the slip.
Robie visits a restaurant. The staff are his old gang from his French Resistance days, paroled based on patriotic war work as long as they keep clean. Bertani, Foussard, and the others blame Robie, because they are currently all under suspicion while the new Cat is active. Still, when the police arrive at Bertani's restaurant, Foussard's teenage daughter Danielle (Brigitte Auber), who has a crush on Robie, spirits him to safety.
Robie can prove his innocence if he can catch the new Cat in the act. He enlists the aid of an insurance man, H. H. Hughson (John Williams), who reluctantly obtains a list of the most expensive jewelry owners currently on the Riviera. Widow Jessie Stevens (Jessie Royce Landis) and her daughter Frances (Grace Kelly) top the list. Robie strikes up a friendship with them. Jessie's 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.




From the trailer






Grace Kelly and Cary Grant






Grace Kelly as Frances Stevens






Kelly and Grant
Frances sees through Robie's cover as an American industrialist. She seduces him, dangling before him her jewels, 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. Robie is accused by Frances of being merely 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, who loses his footing and tumbles over a cliff. 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 presence of the abashed Hughson, 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 the masquerade ball the coming weekend, when he believes the Cat will strike again.
At the ball, Frances is resplendent in a gold gown, Robie 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 get her "heart pills", Robie's voice tips off his identity to the authorities. Upon his return, the police wait out Frances and the Moor as they dance together all night. Finally, Frances and the Moor go to her room, and the mask is removed: it was Hughson, a switch to conceal Robie's exit.
On the rooftop Robie lurks. His patience is finally rewarded when he is joined by another figure in black. But 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 manages to corner his foe with jewels in hand. Unmasked, his nemesis turns out to be Foussard's daughter, Danielle. She slips off the roof, but Robie grabs her hand before she can fall. He forces her to confess loudly to the police of the father-daughter involvement, and that Bertani was the ringleader of this gang.
Robie speeds back to his villa, and Frances races after to convince him that she has a place in his life and home. He agrees, but seems less than thrilled when she says her mother will live with them.

Question: Who was the ringleader of the gang?
Answer: Bertani