Living quietly under the assumed name Paul Sernine, reformed gentleman crook Arsene Lupin is summoned to protect his invalid, wealthy friend Henry Forbes. Despite the watchfulness of Paul and Alexandre Mazeroux, a fellow criminal turned detective, Forbes is murdered. French detective Jabot and the New York detective force have many suspects including Paul, Marie Forbes, the dead man's widow who is suspected on the basis of an apple found with an imprint of her teeth on it, Gordon Savage, her lover, and Florence Chandler, Forbes' secretary and beneficiary under his will. After a series of traps, false arrests, chases through secret passageways, and an escape from handcuffs, Paul deduces that the true criminal is Doctor Varney, who cared for Forbes. Paul prevents Varney from blowing up the house, and at the same time wins the affections of Florence.