Shots are exchanged between police and criminals, who have just robbed a bank. Commissioner Guerchard is responsible for the case. An obviousness according to him: the blow was committed by Arsène Lupin in person! The prefect of police has a plan: to make believe that the investigation is always carried out by Guerchard, whereas it is entrusted in secret to a large policeman whom Lupin does not know. The man in question is named Victor, from the Brigade Mondaine.
