Dr. Kenzo Tenma is a brilliant Japanese neurosurgeon working in Düsseldorf with a glittering future ahead of him. However, his life takes a dark turn after he makes a fateful ethical decision: he chooses to operate on a young boy, Johan Liebert, who arrived with a gunshot wound to the head, instead of the town's mayor who arrived later. Tenma saves the boy, but the mayor dies, costing Tenma his social standing and career prospects.*

Nine years later, the shadows of that night return when Tenma discovers that the boy he saved has grown into a charismatic, cold-blooded serial killer—a "monster" responsible for a string of horrific murders across Germany. Riddled with guilt for having saved such a monster, Tenma abandons his medical career to become a fugitive, embarking on a cross-country hunt to rectify his mistake. As the mystery of Johan’s past unfolds, Tenma is drawn into a complex web of psychological warfare, Cold War secrets, and the terrifying truth behind the nature of evil.