Policemen on patrol in a dangerous area of Paris hear a cry coming from the Poivrière bar and go to investigate. There is evidence of a struggle. Three dead men are lying on the floor, and a wounded man, who is certainly the murderer, stands in a doorway. He tries to escape, but gets caught. Senior inspector Gévrol thinks that the case is straightforward - a pub brawl that ended in murder, whereas his young colleague Lecoq thinks that there is more to the affair than meets the eye. Under interrogation, the suspect maintains that he is an acrobat named Mai, but Lecoq believes he might be the Duke of Sairmeuse and that he doesn't want his identity to be revealed, for his family pride.