At a party thrown at the Metcalf estate, the Marquise D'Irancy's Sultana diamond disappears when the lights go out during a power failure. Suspected of the crime is William Kirkland, the wastrel son of the wealthy Kirkland family, but William's sister Diana comes to his defense. Aiding her in the investigation is Clamp, a wandering peddler. After several misadventures, Clamp reveals that he is a secret service agent on the trail of the international criminals, the de Vallignacs, who are summering at the resort. After proving that the de Vallignacs have stolen the diamond, Clamp arrests his prey, clears William and marries Diana.