Professor Corso Donati, now retired, recalls with a shiver the most inauspicious memory of his career: the school trip to the Aosta Valley that a ruthless murderer has transformed into a real his own slaughter, inspired by the principle of Dante's retaliation and the terrible penalties of the damned described in Hell. A chain of vendettas destined to close, in the killer's mind, precisely with the death of Donati.