Baron Nathan Cahorn receives a letter from Arsène Lupin, who is incarcerated in La Santé Prison, wherein the thief tells Cahorn to send him several of his valuables or else he will come on 27 September to steal those named and more. Cahorn seeks out detective Ganimard, who happens to be on vacation in town, and hires him and two of his men to guard the belongings on the announced date. When the crime occurs, Ganimard asks Cahorn not to tell people he was there and an official investigation is launched, during which Ganimard is called in as the expert on the thief. Ganimard goes to see Lupin in prison, where the thief explains it was he who was hired to watch the night of the crime. Lupin also states that he was only arrested because he was distracted by a woman he loved and declares that he will not be present at his own trial.