The funds of Lorenzo Corcini, an international thief, are getting low. With his wife, Clarice, he is living at Nice, the most fashionable resort on the Riviera. Suddenly there comes a chance to recoup his fortune by robbing Lady Stanley. She is the possessor of a beautiful sapphire necklace. By forging a letter of recommendation Corcini is able to get his wife introduced into the household of the wealthy woman as a governess. Upon assuming her duties, Clarice discovers that Lady Stanley is in love with Lord James Pearce. The night chosen for the theft is ideal for Corcini. Lady Stanley has been engaged with Lord Pearce all evening. Quite unobserved, the thief is able to make his way into her room with the aid of his accomplice. He is about to secure the gems when Lord Pearce himself appears, having come for a coat for Lady Stanley. He throws aside his own automobile coat and forgets it in his haste to take the warm coat to the woman he loves. Corcini breaks the gold string which holds the sapphires and slips two of the jewels into Lord Pearce's coat. He then makes his escape. When Lady Stanley discovers her valuable necklace has been stolen, there is an immediate outcry. The two jewels are found in Lord Pearce's coat, and the examining magistrate is reluctantly forced to accuse him. However, Lady Stanley immediately employs Detective Kips to uncover the real criminal. Clarice, still masquerading as the governess, learns that Kips is on the job and warns Corcini. Kips discovers a clue that leads to Corcini and has the latter's house watched. It is the carnival season at Nice, and the thief escapes from his home, taking refuge in one of the handsome floats which will figure later in the day in the parade. He overcomes one of the men on the float who leaves the platform during the parade, and dresses in his Pierrot costume. Thus disguised he manages to slip to the ground and into the carnival crowd. He is overcome with horror when he finds that the necklace has been removed from its hiding place, but takes courage when he finds a note in code from Clarice saying that she has the necklace, because she fears the police will make a search. Kips and Lady Stanley follow and find that Clarice is met by Corcini. The criminal, recognizing that it will be difficult for him to escape when hampered by a woman, has decided to get rid of Clarice. This he does at the ball by means of poison. However, he is already under surveillance and is unable to escape. Arrested by Lady Stanley herself, who had trapped him into waltzing with her, the man is taken away to prison. The necklace is found in his possession. The cloud which rested upon Lord James Pearce is lifted, and all ends happily.