Paris, 1893. An enamel artist was stabbed to death, the bookbindery burned down, and the owner of the printing house disappeared without a trace. Three sad events at first glance are in no way connected, but each time someone leaves mysterious messages at the crime scene, in a newspaper or in an envelope, which mention the month of May and a leopard. Bookseller Victor Legris and his faithful assistant Joseph begin a new investigation...