Arsène Lupin decides to run a detective agency in addition to being a gentleman thief. As a detective he happens to cooperate with police in order to unveil the criminal activities of a villain. When he succeeds the villain returns the favour. The unmasked Arsène Lupin manages to escape with the villain's gangster moll as his new companion.

Arsène Lupin is extremely popular among the population, because he allows the needy to share in his acquired wealth. Before entering the service of the German Emperor Wilhelm II, he removes his vault in Alsace, steals two paintings of old masters, steals valuable gems and calls out to the police prefect to avoid his arrest. But this time he risks being seriously recognized. Lupin must once again use his fine intellect to deftly escape the situation.

At the end of the Great War, Arsène Lupin resumed his adventurous life. Theft of three paintings committed by an adversary leads him to the treasure of the Golden Fleece.

André Laroche, an industrialist, has just passed away. Face to his grave, as he is being buried, Anne de Vierne, the wife of a magistrate, confesses to her son François that Laroche was in fact Lupin and that he is his natural child. But he is not the gentleman thief's only offspring! Lupin had indeed another son by a housemaid, Gérard Dagmar, a dancer, magician and - occasionally - burglar. Which complicates the task of François who, to respect the last wishes of the testator, has gone in search of the treasure of Poldavia. For he keeps finding Gérard on his way and his efforts are constantly thwarted by his half-brother. Will Gérard prevent François from becoming the worthy successor to their father or will the two young men decide to join forces? That is the question.

As the daring thief Arsène Lupin ransacks the homes of wealthy Parisians, the police, with a secret weapon in their arsenal, attempt to ferret him out.