Michael Lanyard, the retired gentleman crook known as the Lone Wolf, is puttering with his aquarium when his old friend, Stanley Young, appears to enlist his aid in recovering a priceless pearl necklace that has been stolen from his murdered friend, Phillip Jordan. Young tells Lanyard that Jordan had found out that Binnie Weldon and her accomplice, Jim Ryder, stole the pearls and replaced them with fakes, and Lanyard agrees to switch the pearls back again. He is hampered in his task, however, by the misguided meddling of Delia Jordan, the murdered man's daughter. Posing as foreign fence Emil Gorlick, Lanyard obtains the pearls from Binnie and Ryder, but after he turns them over to Stanley, his old friend is found murdered and the pearls are discovered missing. Stanley's murder throws suspicion on Lanyard, and to clear himself of the crime, he must find both the murderers and the necklace. To accomplish this, Lanyard tricks the killers into believing that they have the fake pearls and Delia has the real ones. Much to Delia's dismay, Lanyard's trap nets her, suitor Ralph Bolton, and Alberts, the man who hired Bolton to keep an eye on the pearls. After convincing Alberts that he has the genuine pearls, Lanyard leads the killers on a merry chase, which ends on a New York ferryboat where the police await them.