While at the end of the 19th century, a bloodthirsty killer, described as a "vagabond" in the cities where he rages, crisscrosses France, judge Emile Fourquet examines the unresolved case of the little Victor Portalier. After a long work of profiling, he manages to identify the culprit: a certain Joseph Vacher, who recognizes all the murders. This case marks a turning point in the history of the French police: it was in reaction to the ineffectiveness of his men on this file that Georges Clemenceau created the famous "Tiger Brigades".