Monsieur Lecoq is a captivating mystery, historical and love story: Around 11 o'clock, on the evening of Shrove Sunday 18.., close to the old Barrière d'Italie, frightful cries, coming from Mother Chupin's drinking-shop, are heard by a party of detectives led by Inspector Gévrol. The squad runs up to it. A triple murder has just been committed. The murderer is caught on the premises. Despite Gévrol's opinion that four scoundrels encountered each other in this vile den, that they began to quarrel, that one of them had a revolver and killed the others, Lecoq, a young police agent, suspects a great mystery...