Countess Lily is vacationing in Venice. Except that Lily is not really a countess. Even worse: she’s actually a pickpocket. While in Venice, Lily meets a tempting target: Baron Gaston Monescu. While enjoying a sumptuous dinner at the hotel, Lily—with the finest etiquette one might expect from a countless—accuses Gaston of being a thief. To which he immediately retorts with the accusation that she is a mere picker of pockets. Turns out both are right, but that Lily is perhaps less light-fingered than she thought.
Gaston marked her when she tickled him while making a lift from his pocket. Lily inquires about the time and then shows Gaston his watch. Gaston counters by exhibiting her own garter. It is love at almost first sight among thieves.
Gaston is posing as a doctor when he steals from the aristocratic Francois Fileba and then escapes with Lily in tow. Not quite a year later, the love is still on the bloom between the two as they enjoy an opera in Paris. Mixing business with pleasure, Gaston coolly makes off with the purse of Mariette Colet, a perfume tycoon. The purse just so happens to be studded with sparkling diamonds. Later, in the guise of a member of the “nouveau poor” Gaston collects 20,000 franc reward for returning the purse to its owner while at the same using his robust amount of charm to worm his way into a position as her private secretary.
The target of his next big heist is the 100,000 francs that Mariette keeps in the safe inside her home. Lily is recruited into the plan as Gaston’s assistant, the dowdy Mlle. Votier. Mariette Colette appears to view Gaston as potentially much more than a mere secretary, which drives Lily crazy, but she nevertheless remains committed to the scheme by playing the part of the devoted employee. Trouble in paradise arrives in the form of Gaston’s recent target Francois Fileba who just so happens to be among the social circle which Mariette calls her own. Fileba is sure he knows Gaston from somewhere and even inquires if he’s ever been to Venice. The situation gets even worse when Monsieur Giron, the Chairman of the perfume company Mariette owns, suddenly accuses Gaston of embezzling. The dual pressure bearing down hard upon them, Gaston and Lily makes plans to leave Paris prematurely.
Unbeknownst to Lily, Gaston is romancing with Mariette while she packs. Meanwhile, Fileba has finally recalled where he has seen Gaston and tries to warn Mariette, but she will nothing of it. Giron then shows up to not only accuse him of stealing from the company, but of being the infamous thief Monescu. Gaston is able to rid himself of Giron’s pestering by turning the tables and accusing Giron of stealing from Mariette.
Lily finally catches on to the fact that the sudden postponement of their plan to get out of town quickly is due to Gaston’s plan to rendezvous with Mariette. Scornfully emasculating him by addressing him as Monsieur Colet, Lily then steals the money from Mariette’s safe herself and informs Gaston that it wasn’t him, but the cold hard cash she wanted all along.
Mariette returns and is just about to put a pearl necklace into her safe when Gaston tells her that it was Giron who has been embezzling. Shortly thereafter, he suddenly confesses that it is true that he is the infamous thief Monescu and, furthermore, he has stolen the cash she keeps stashed in the safe. He then says that even though his plan was merely to rob her, something quite unexpected happened: he fell in love with her.
Just then, Lily interrupts them by confessing to being the one who actually stole the money. Then she makes an indecent proposal: Mariette can make love with Gaston…in exchange for the full 100,000 francs. Gaston replies to this by suggesting that it would be better for the business interests of all concerned if he stayed faithful to Lily. He then bid adieu to Mariette who responds by saying their romantic interlude would have been divine. She also allows Gaston to leave with the pearl necklace and present to them to Lily who has always admired and desired them. Outside in the cab, Gaston reaches for the necklace to give it to Lily only to find that she has already picked it from his pocket. Not only does Lily have the necklace, she also has that diamond-studded bag which started the whole thing. What Lily does not have is the 100,000 francs. But this absence is very short and very sweet as Gaston hands the cash he has stolen from her back into her hands before they kiss.