Kenzo Tenma is a brilliant Japanese neurosurgeon living in Germany whose life changes when he chooses to save a young boy instead of an influential patient. Years later, he discovers that the child he saved has become an extremely dangerous criminal. Consumed by guilt, Tenma gradually leaves his career behind and sets out to find him in order to understand what he has become and prevent further victims. His investigation takes him across Europe, uncovering a complex past, long-hidden manipulations, and people deeply scarred by violence and secrets.