The Lone Wolf, an international thief who is expert in dodging the police, is rifling a wall safe when he senses the approach of detectives. Secreting a necklace in a cigarette case, he makes a hasty retreat. In disguise, he climbs to the balcony of another house where a bal masque is in progress and conceals himself in Marcia Mayfair's boudoir. Later he mingles with the guests and dances with Marcia. Lanyard is disclosed as a stranger when detectives force the guests to unmask, but Marcia remains silent while Lanyard replaces the jewels lifted from her bedroom. Lanyard is warned by Detective Crane, who suspects him but has no evidence. A visit to a bohemian resort owned by Morphew, leader of a band of thieves, provokes a heated argument in which Lanyard is threatened; and Marcia and Lanyard barely escape being caught in a raid. Their romance is threatened when Marcia's jewels are again stolen, but Lanyard traps Morphew and his gang and brings them to justice.