This time, however, Valeria isn't there to question those close to the victim as she is in Rome regarding her imminent transfer. Andrea Russo, her right hand man, sees to it, not as talented but definitely just as interesting. To convince Valeria to stay Lombardi promises that they will reopen the case of her father's murder.

A university math professor, Emanuele Sarti, is found dead in a tunnel inhabited by a homeless man in Valentino Park. Strange mathematical formulas are discovered written on the walls of the tunnel: improbable that it's a coincidence.

After deciding to remain in Turin, Valeria returns to the crime scene of her father's murder and discovers a piece of evidence that has been overlooked in the past: that night, in the house on the lake, in addition to her mother and father, a third person has been present.