Dr. Kenzo Tenma is a brilliant Japanese neurosurgeon working in Germany, with a promising career and a beautiful fiancée. His life is upended when he chooses to save the life of a young boy, Johan Liebert, instead of the city's mayor, following his moral compass rather than hospital politics. Years later, Tenma discovers that the boy he saved has grown into a cold-blooded, sociopathic serial killer—the "Monster" of Germany. Burdened by the guilt of his own mercy, Tenma abandons his medical career and goes on the run as a murder suspect himself, traveling through a post-Cold War Europe to track down Johan and stop the trail of blood he started.