In 1919, a serial killer, the Axeman, has New Orleans hiding behind doors. Then the killer issues a warning: on a named night, he will spare every person in a home where a jazz band is in full swing; all others risk getting the axe. Police detective Michael Talbot is struggling to find leads. His former boss, Luca d'Andrea, out after six years in the penitentiary, is looking just as hard but working for the Mafia, which needs to find the killer just as much as authorities but for a different reason. Meanwhile, Ida, a secretary at the Pinkerton National Detective Agency, using the methods of her idol, Sherlock Holmes, discovers a clue that leads her and her musician friend, Louis Armstrong, into the case, and into danger.