The medieval land of Parthoris is swept by a mysterious plague. Its infected inhabitants descend into madness. Corvus, the protagonist of the first Heretic, finds a way to escape from his home town, but is still contaminated by the plague. Since he possesses one of the ancient Tomes of Power, he is not immediately affected by the plague; but sooner or later, he will have to succumb to it. Before that happens, Corvus must find who or what was causing the plague in the first place...