At 8:05 pm, Lily phones for Gaston/LaValle, but he doesn't respond to Jacques' ring. At 8:30 pm, Jacques knocks on the office door - first, Gaston/LaValle emerges from her bedroom door, and then she emerges from her bedroom door:

Gaston/LaValle: Jacques, dismiss the car, Madame is not going.
Jacques: Yes, Monsieur LaValle.
Mme. Colet: Jacques...don't dismiss the car. I'll be down in a few minutes...
Gaston/LaValle: I want you to stay, Mariette. You've got to stay. You can't go now.
Mme. Colet: I must go.
Gaston/LaValle: I'm crazy about you.
Mme. Colet: I know it.
Gaston/LaValle: I love you.
Mme. Colet: I believe you.
Gaston/LaValle: Then why do you want to go?
Mme. Colet: Because I want to make it tough for you.

Gaston/LaValle passionately embraces her, as love rears its head and forces him to succumb to her stylish seduction. Their reflection is framed in a round mirror above the bed. As Gaston's passion escalates, Mme. Colet makes suggestive remarks - telling him that they have lots of time to consummate their love. Their romantic images are frozen and framed in static images that include reflections or silhouetted shadows - suspending them in a paradise of time for an eternity. She retreats precisely because of his embraces and advances:

We have a long time ahead of us, Gaston. Weeks...(A closer medium range shot frames them in a dressing table mirror) ... Months... (Their shadows are reflected on her bed) ... Years (The two sexy silhouettes kiss.). (As she leaves the room for the dinner party, she turns and caresses the door in a languid way) Eleven o'clock.

In a one-sided phone conversation, Lily is told that Gaston has changed his mind about leaving on the midnight express train: "What!?...Tomorrow morning? Why?...Of course, uh-huh, uh-huh." Compelled to join forces, the two rival suitors at the party notice Mme. Colet's emotional swooning (for Gaston) and casually denigrate their mutual rival:

Major: No doubt about it. It's that secretary.
Filiba: Funny the kind of men women fall for.
Major: No color, no sparkle, but dependable.
Filiba: The type they marry.
Major: You know, I'm not the marrying type. I like to take my fun and leave it....
Filiba: You know, he's really not such a bad fellow.
Major: No, just dull.
Filiba: Insignificant. He's a secretary. He always was a secretary, always will be.
Major: That's funny, heh, heh. The first time I saw him, I thought he was a doctor.

The hearing of the word doctor triggers another associative memory for Filiba - at the moment of recognition, the soundtrack music spontaneously emphasizes his realization. He approaches Mme. Colet to exclaim:

Tonsils! Positively tonsils!

At the same moment of Filiba's discovery, Monsieur Adolphe Giron confronts Gaston as "Monsieur Monescu" and threatens to call the police unless he evacuates Mme. Colet's mansion by the next morning. But the wily Gaston counter-accuses the trustworthy accountant Giron of being a crook himself for fraudulently embezzling the perfume company's funds. His knowledge and exposure of Giron's long-standing crime protects him (and Giron promptly leaves):

Gaston: You say I'm a crook.
Giron: I know it!
Gaston: Then why didn't you call the police? Why don't you call the police? I'll tell you why, you crook you!
Giron: (trembling) Monsieur -
Gaston: Monescu.
Giron: Monsieur Monescu.
Gaston: Just call me Gaston.

But his sly 'stealing of love' from Mme. Colet is not as easy to dismiss. In a dramatic scene, Gaston meets Lily in front of Mme. Colet's wall safe where she demandingly insults him for his calculated romantic fraud and infidelity:

Gaston: Are you insane? You have to get out of here at once! She may be back at any minute.
Lily: What time is your rendez-vous?
Gaston: Now, Lily -
Lily: Yes, Monsieur Colet.
Gaston: You have to get out of here!
Lily: That's what I'm here for - to get out. I want to get away from here, from you, just as fast as I can and as far as a hundred thousand francs will take me... (She dials the wall safe) 65 ... 35 to the left ... I wouldn't fall for another man if he were the biggest crook on earth ... 76 ... 84 ... What has she got that I haven't got?
Gaston: You must listen to me -
Lily: Shut up! Don't make up any stories!
Gaston: But Lily -
Lily: Don't you dare lie to me! I know you love me. Oh, why don't you say something? Come on - be brilliant! Talk yourself out of it - bluff yourself in!
Gaston: (He starts to say something)
Lily: Shut up, you liar, you! (She swings the safe door open and takes out a wad of banknotes - a hundred thousand francs in total.) That's what I want! This is real! Money! Cash!

After Lily descends the spiral staircase and leaves with the money, Mme. Colet returns home from her dinner engagement, where she has been told that Gaston was the Venezian thief who robbed Filiba. Upstairs in Gaston's presence in his bedroom, Mme. Colet starts disrobing - first removing her pearl string necklace, bracelet jewelry, and rings:

Mme. Colet: When a lady takes her jewels off in a gentleman's room, where does she put them?
Gaston/LaValle: On the, on the night table.
Mme. Colet: But I don't want to be a lady.

Mme. Colet begins opening the wall safe to place her jewels there. Characteristically, Gaston attempts to distract her, and then boldfacedly exposes Adolphe's robbery of the Colet Company funds, protecting her from criminal forces in her own trust. At first she is disbelieving:

Gaston: What would you say if you found your safe had been robbed?
Mme. Colet: I wouldn't say anything. I would act.
Gaston: Call the police?
Mme. Colet: Instantly...But why talk about robbery on a night like this?
Gaston: You look beautiful.
Mme. Colet: Thank you. (She continues to dial the combination)
Gaston: Mariette!...You have been robbed - for years. And not a hundred thousand francs, but millions. And you know who did it? Adolphe.
Mme. Colet: Adolphe?
Gaston: Adolphe J. Giron.
Mme. Colet: And you expect me to believe that?
Gaston: Naturally not. But I expect the police to believe it.

When he picks up the phone to notify the officials, she persuades him otherwise. Gaston reveals his own true identity as a "self-made crook" - devastating and crushing Mme. Colet's love for him with his genuine honesty:

Gaston: I see! You have to be in the Social Register to keep out of jail. But when a man starts at the bottom and works his way up, a self-made crook, then you say, 'Call the police! Put him behind bars! Lock him up!' Very well, Madame. I am Gaston Monescu. The police will be delighted to verify my identity. (He offers her the phone to report him, but she declines.)
Mme. Colet: Gaston, did you take the money?
Gaston: Yes.
Mme. Colet: You wanted a hundred thousand francs. And I thought you wanted me. (She moves to a window, where a bell tower is seen in the distance and chimes are heard.)
Gaston: I came here to rob you, but unfortunately I fell in love with you. Mariette?
Mme. Colet: Why did you take the money? (He doesn't answer.)

In the privacy of her own room, Lily approaches Mme. Colet. Jealous of her partner's infatuation with the wealthy widow, Lily regains the exclusive love of Gaston. She admits to robbing her alone and decides to ditch her unfaithful partner:

Lily: Madame, the only thing that seems to stand between you and romance is a hundred thousand francs. Well, he didn't take it. (She opens her purse to reveal the wad of 100,000 franc banknotes.) I took it - all by myself. Now you can have your romance.
Mme. Colet: I think you'd better go.
Lily: Ever had a romance with a crook?
Mme. Colet: I beg your pardon?
Lily: Let me give you a little advice. When you embrace him, be sure to put on gloves. It would be too bad if your fingerprints were found...
Mme. Colet: Mademoiselle Vautier, or whatever your name is. I thank you for your free advice but I must ask you to go. You have your money.
Lily: I don't want your money! (Monescu enters the room) You wanted to buy him for 50 francs. Well, you can have him for nothing! (Lily throws the banknotes onto Mme. Colet's bed.) (To Gaston) And you!
Gaston: Lily!
Lily: Leave me alone. You were willing to sacrifice a hundred thousand francs for her! (To Mme. Colet) And you! You paid a hundred and twenty-five thousand francs for a handbag. Well, you can pay a hundred thousand for him! (Lily retrieves the 100,000 francs in banknotes.) Goodbye Mme. Colet (and with a sneering glance toward Gaston) and Company! (Lily departs and slams the door.)

Gaston pursues her, realizing that he is truly in love with Lily. Mme. Colet hears their footsteps through the door as they scamper down the long staircase. Momentarily, Gaston returns and regretfully bids Mme. Colet a final goodbye, advising her to look carefully at her illusory life and acknowledging that his arrest is imminent. She sadly but willingly lets him part as a romantic partner and surrenders her necklace as a gift to Lily:

Gaston: Goodbye.
Mme. Colet: (She extends her hand for a handshake.) Goodbye.
Gaston: You could have been marvelous.
Mme. Colet: Divine.
Gaston: Wonderful...But tomorrow morning, if you should wake out of your dreams and hear a knock, and the door opens, and there instead of a maid with a breakfast tray stands a policeman with a warrant, then you'll be glad you are alone.
Mme. Colet: But it could have been glorious.
Gaston: Lovely.
Mme. Colet: Divine...But that terrible policeman. (He agrees with a shake of his head)
Gaston: Goodbye. (They kiss) (At the door, he turns) Do you know what you're missing? (She nods) No. (He pulls her string of pearls from his coat pocket.) That's what you're missing. Your gift to her.
Mme. Colet: (graciously and with composure) With the compliments of Colet and Companie.

In the film's epilogue as the film fades, the high class, amoral thieves Gaston and Lily escape in the back of a taxi-cab, now more deeply aware of their larcenous, lower-class criminal affinity for each other. Searching frantically in his pockets, Gaston discovers that Lily has already pickpocketed the pearl necklace. She reveals the pearls from inside her dress and places them in Mariette's purloined 125,000 franc handbag sitting on her lap. To her dismay and delight, she holds her purse open, while Gaston politely shoves the wad of 100,000 franc banknotes that he has stolen from her (Lily) back into her purse. Their separate hauls from the widow - (some acknowledged and some unknowingly surrendered) - are now combined in their happy reunion.

Back to their old ways, reminiscent of the earlier scene of their mutual thefts in Venice, they lovingly embrace each other as Lily passionately cries out: "Gaston!" - en route to new conquests in an uncertain world.