A charming and very daring thief known as Arsene Lupin is terrorizing the wealthy of Paris. He even goes so far as to threaten the Mona Lisa. But the police, led by the great Guerchard, think they know Arsene Lupin's identity, and they have a secret weapon to catch him.

A woman and a man vying for a woman's affection: the usual love trio? Not quite so since the belle in question is Lorraine de Grissac, a very wealthy and alluring society woman, while one of the two rivals is none other than Arsène Lupin, the notorious jewel thief everybody thought dead, now living under the assumed name of René Farrand. As for the other suitor he is an American, a former F.B.I. sleuth turned private eye by the name of Steve Emerson. Steve not only suspects Farrand of being Lupin but when someone attempts to steal a precious emerald necklace from Lorraine's uncle, Count de Brissac, he is persuaded Lupin is the culprit. Is Emerson right or wrong? Which of the two men will win over Lorraine's heart?

A rich but naive young woman is in possession of some priceless jewels. She herself doesn't know it, but a gang of jewel thieves does.