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 fathers 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.
