In the famous seduction scene in Frances' hotel suite, she has invited him to join her and watch the fireworks. The real fireworks explode through the open doors in the background (over the water in the night sky), as other 'sexual' fireworks burst within the room. The metaphoric dialogue is exceptionally laced with playful sensuality. At one point, she stands with her face in the shadows as she describes a thwarted theft. On the couch, she forces Robie into admitting his passion for her. Acting as a exploitative predator, she entices and lures him into her arms by displaying her white strapless gown and his main weakness - her sparkling diamond necklace (and her bare decolletage and breasts) as the ultimate prize. [Earlier, she had stated that she didn't like 'cold' diamonds touching her skin.]

The scene climaxes with an intercut montage of explosions of roman candles bursting in the night sky (and coming closer and closer into view) while she seduces him by plotting the robbery of the Sanford villa with him ("we'll do it together"). She gets him to admit that he is a thief, and then encourages him to extol the beauty of both her diamonds (imitation) and her breasts ("Hold them"); in fact, she equates her sexuality as something valuable to be robbed:

Frances: If you really want to see fireworks, it's better with the lights out. (She turns off the lamps in the room one by one.) I have a feeling that tonight, you're going to see one of the Riviera's most fascinating sights...I was talking about the fireworks.
John: I never doubted it.
Frances: The way you looked at my necklace, I didn't know. You've been dying to say something about it all evening. Go ahead.
John: Why, have I been staring at it?
Frances: No, you've been trying to avoid it.
John: May I have a brandy?
Frances: Please.
John: Do you care for one?
Frances: No thank you. Some nights a person doesn't need to drink. Doesn't it make you nervous to be in the same room with thousands of dollars' worth of diamonds and unable to touch them?
John: No.
Frances: Like an alcoholic outside of a bar on election day.
John: (He laughs) Wouldn't know the feeling.
Frances: All right. You've studied the layout, drawn your plans, worked out your timetable, put on your dark clothes with your crepe-soled shoes and your rope. Maybe your face blackened. And you're over the roofs in the darkness, down the side wall to the right apartment, and the window's locked. All that elation turned into frustration. What would you do?
John: I'd go home, get a good night's sleep.
Frances: Oh, what would you do? (She steps into the darkness that hides only her face.) The thrill is right there in front of you, but you can't quite get it - and the gems glistening on the other side of the window, and someone asleep, breathing heavily.
John: I'd go home, get a good night's sleep.
Frances: Wouldn't you use a glass cutter, a brick, your fist - anything to get what you wanted? Knowing it was just there waiting for you?
John: (He sips his brandy.) Oh, forget it.
Frances: Drinking dulls your senses.
John: Yeah, and if I'm lucky, some of my hearing.
Frances: (She fondles her own necklace.) Blue-white with just hairlike touches of platinum.
John: You know, I have about the same interest in jewelry that I have in politics, horseracing, modern poetry, or women who need weird excitement: none.
Frances: Hold this necklace in your hand and tell me you're not John Robie, 'the Cat.' John, tell me something. You're going to rob that villa we cased this afternoon, aren't you? Oh, I suppose 'rob' is archaic. You'd say, 'knock over'?
John: Oh -
Frances: Don't worry, I'm very good at secrets.
John: Tell me, have you ever been on a psychiatrist's couch?
Frances: Don't change the subject. I know the perfect time to do it: Next week, the Sanfords are holding their annual gala. Everyone who counts will be there. I'll get you an invitation. It's an 18th-century costume affair. There will be thousands upon thousands of dollars' worth of the world's most elegant jewelry. Some of the guests will be staying for the weekend. We'll get all the information, and we'll do it together. What do you say?
John: My only comment would be highly censorable.
Frances: (She sits alluringly on the couch, displaying both her necklace and bare decolletage.) Give up, John. Admit who you are. Even in this light, I can tell where your eyes are looking. (He sits down.) Look, John. Hold them. Diamonds. The only thing in the world you can't resist. Then tell me you don't know what I'm talking about. (She kisses his fingers, one by one.) (She puts her necklace in the palm of his hand.) Ever had a better offer in your whole life? One with everything?
John: I've never had a crazier one.
Frances: Just as long as you're satisfied.
John: You know as well as I do. This necklace is imitation.
Frances: Well, I'm not. (They kiss. The white-hot peak of the colorful fireworks exhibition bursts in a vibrant closeup.)

Later, she sleeps on the couch (after 'doing it' and losing her virginity? - often called "the family jewels") while he goes out onto her open balcony. After some time passes, he nervously sits alone in his room on a lower floor of the hotel, watching anxiously for the Cat. Suddenly, she opens his door, casting harsh light on him. She tearfully accuses him of being responsible for the loss of her mother's jewels (and her own sexual loss of virginity): "Give them back to me...Mother's jewels!" She demands their return. When he denies having them, they struggle together, and then she informs him that the burglary occurred while she was "asleep." He leaves her to search in his room, while he goes to Mrs. Stevens' room to look around.

He bluntly confesses his real identity to her: "My name is John Robie. I used to be a jewel thief several years ago." She is stunned - and thrilled as her daughter once was: "Well, what a wonderful surprise!" and she continues to watch him in astonishment as he inspects everything and deduces that the cat burglar came down through the air shaft. Francie enters and cautions her mother about speaking to him (he's "a notorious jewel thief called the Cat"), and she produces Hughson's list of jewel-owning clients from his room as evidence. She tells him that he's "already caught" since she has phoned the police and told them "everything." Mrs. Stevens is unconvinced that he is guilty and believes his denials - otherwise, why would he be "returning to the scene of his crime?" She responds to her frivolous daughter: "Since when is love a crime?" She commends Robie on being a "swindler" and a "real man. Not one of those milksops you generally take up with."

Mrs. Stevens: Why do you think moved so many times, hmm? Your father was a swindler, dear, but a loveable one. If you ask me, this one's a bigger operator on every level.
John: Thank you, madam.
Francie: Mother, this is why I've had to spend half my life traveling around the world after you, to keep men like this away from you.
Mrs. Stevens: Well, after this, let me run my own interference. Looks like the blockers are having all the fun.
Francie: If she doesn't have any common sense, I do.
Mrs. Stevens: Oh, shut up! They were my baubles that were stolen. If I don't care, why should you? They're insured.

Mrs. Stevens is unmoved by the theft of her jewels. When the police arrive, Robie slips out unseen to the roof and escapes, while Mrs. Stevens covers up and ignorantly claims that she has "never heard of him." Later, as they circle around a sofa and exchange insults, Mrs. Stevens continues to argue that the 'American' is innocent until proven guilty. She chastises her adolescent-minded daughter for having the wrong upbringing as an heiress:

...you ought to be spanked with a hairbrush and sent back to school - public school, where they could pound some sense into you during recess.

When Francie calls Robie "a low worthless thief," her mother cryptically asks about what he may have stolen from her (sensing that she has just lost her virginity): "Just what did he steal from you?"

In four short scenes in the following montage, the news of the most recent crime spreads at a press conference in the police station, at Bertani's restaurant, at Robie's villa, and at the Carlton Hotel beach. At a fishing wharf, Robie, disguised as a fisherman (wearing a vest, sunglasses and hat), meets with Hughson with another plan to catch the cat burglar. He has been watching the villa of the Silva's, a South American couple who are insured and on Hughson's list (with jewels valued at $160,000), and he has noticed that "somebody else is watching it too" in the dark. His plan is to set a "foolproof trap" for the imitative burglar that evening, and have Commissioner Lepic's men alerted and present. In fact, however, the trap backfires - Foussard and other staff from Bertani's Restaurant lie in wait to seize Robie. Shockingly, after he is jumped from behind in the dark and a wrench is raised high in the air, one figure falls to his death from the top of a wall. [Robie escapes.] The police discover that the victim is Danielle's father, the wine steward Foussard. The newspapers mis-broadcast the news that Foussard was the Cat burglar: "LE CHAT EST MORT" (the "Cat" is dead).

In Lepic's office, Robie informs the Commissioner (and Hughson) that Foussard was not the cat burglar, because he had a wooden leg (from the war) and would have lacked the agility "of a four-footed cat" to crawl over slippery rooftops. They have reported the death only because of the resultant positive publicity: "Newspapers have their headlines, all the rich tourists can relax, and you Lepic got your publicity and possibly your commendation from the Paris office."

At the burial site during Foussard's funeral, Robie expects to get a look at the at-large "real cat - who will certainly be there purring." Danielle contemptuously shouts French epithets at Robie and publically accuses him of murdering her father: "It's because of you he's dead...Get out of here - killer...Murderer." [Similarly, the police suspect that the Cat burglar - or Robie - was Foussard's murderer. It is later revealed, however, that the murderer is Bertani.] Indignant, he slaps her across the face and then pushes his way through the crowd to leave.

Outside the cemetery where a hearse is parked, Francie is waiting in her sports car to apologize for accusing him of being the thief. After telling her that Foussard wasn't the cat, she offers to help him find the truth and then confesses her love for him:

Francie: Let me do something to help you.
John: Oh, no thanks. Now you've just made your apologies. Let's just go back to our mutual disregard of each other, hmm?
Francie: Mr. Robie. I was wrong about you, I think. You might possibly be wrong about me.
John: Well now, that's another thing that I may never know. If you'll pardon me...(He turns to leave - and she grabs his arm.)
Francie: I won't pardon you. I'M IN LOVE WITH YOU.
John: That's a ridiculous thing to say.
Francie: Is it?
John: To you, words are just playthings.
Francie: (pouting) Word playthings.
John: I'll make you a sporting, exciting offer.
Francie: (hurt) I don't know if I'm up to it now.

He asks her to give him an invitation to the costume gala at the Sanford's by accompanying Francie and her mother masquerading in Louis XV outfits. He promises that she will "see a real live burglary in action."

The evening costume ball is attended by rich partyguests wearing expensive jewels - newly-arrived couples walk through a long gauntlet of attendees to show off their costumes and spectacular jewelry. Outside, Danielle (in a French maid's outfit) assists Bertani and the restaurant staff in catering the affair. When the Stevens arrive, Mrs. Stevens is costumed in a navy blue gown while Francie is dressed in a glittering, tight-fitting gold gown. Behind them, Robie trails as a black-masked umbrella-bearer and Nubian slave. He warns them: "Any man without a lady on his arm can only be a policeman." Within Lepic's earshot, Mrs. Stevens tactlessly (but deliberately) identifies John as their disguised servant and asks him to fetch her "heart pills." For the rest of the decadent evening after 'he' returns, he dances (through a samba, then a waltz) the night away with Frances (under constant police surveillance) until the orchestra is commanded to stop playing and they (and the police) are the last to leave.

When they finally retire to their room at the conclusion of the ball, Hughson (not Robie) removes his black-mask. As part of their scheme, he surreptitiously slipped into Robie's costume and exchanged places to allow Robie freedom to track the real cat from the villa's rooftop during the evening. Conducting surveillance, John notices a black-clad figure exiting a window and traversing the rooftop. He freezes, spies the figure, and then chases after it. The police on the driveway hear a crash and are alerted to the activity and movement on the rooftop. Robie catches up to the masked thief - grabs the person - and unmasks DANIELLE.

I figured it was you the night your father died. He couldn't climb anything, and you always did his legwork for him even during the war when you were a kid.

From the courtyard, the police shine a searchlight onto the roof and call out: "Come down, Robie, or we shall be forced to shoot." Danielle breaks his grip and hides, while he 'freezes' in the glare of the light. As she tries to escape, she jumps, trips and slides to the roof's gutter, hanging precariously by one hand. Francie begs the police and Lepic to quit shooting at Robie: "Put that gun down. He's not the Cat." Lepic remarks that he can only believe what he sees: "John Robie's just where I knew he'd be some day." Robie prevents Danielle from falling to her death by grabbing her by one wrist and holding her in mid-air. She drops a bag full of jewels to the ground far below and entreats Robie to: "pull me up." He threatens to drop her unless she fully confesses loudly to the group below. She reluctantly admits her guilt and implicates Bertani as the mastermind behind all the robberies:

I was working for my father...Bertani was behind it.

The final sequence opens with a car chase and squealing tires (John's car, now with him at the wheel and fearing being caught, is being pursued by Frances in a police car) racing to Robie's Cote d'Azur villa. Still wearing her gold gown, Francie follows John onto his veranda. She claims he has been running away from her, and avoided saying a proper thanks and goodbye - especially after she (as his 'girl Friday') saved his life. She turns the tables on him by requesting that he declare his love for her - in a partially-dictated confession:

Frances: Oh, John, you left in such a hurry you almost ran.
John: I had work to do up here.
Frances: Were you afraid to admit that you just can't do everything by yourself and that you needed the help of a good woman? And you just aren't the lone wolf you think you are.
John: All right. Without you, I couldn't have done. I needed the help of a woman. I guess I'm not the lone wolf I thought I was, Francie.
Frances: Well, I just wanted to hear you say that. Thank you. (She extends her hand.) Goodbye.
John: Goodbye. (He pulls her arm toward him for an embrace and kiss.)
Frances: (opening her eyes) So this is where you live. Oh, Mother will love it up here!

After surveying his beautiful villa on the hillside, her final line of reflective dialogue confirms that she too is a thief who has stolen his affection - and his villa, and given him a mother-in-law through marriage. With a side glance, he looks at her with dismay - as a church bell tolls.