Dr. Kenzō Tenma is a young Japanese doctor, working at Eisler Memorial Hospital in Düsseldorf in 1986. An accomplished brain surgeon, he seems to have everything: a soon-to-be promotion; the favor of the hospital’s director, Udo Heinemann; and Heinemann’s daughter Eva as his fiancée. However, Tenma is increasingly dissatisfied with the political bias of the hospital in treating patients, and seizes the chance to change things after a massacre brings fraternal twins Johan and Anna Liebert into the hospital. Tenma, going by his ethics as a doctor, decides to operate on Johan instead of the mayor of Düsseldorf, who arrived later. Johan is saved, but Mayor Roedecker dies. Tenma loses his social standing (and Eva) as a result, but comforts himself by believing he did the right thing and despairs of ever achieving his earlier high goals. However, Director Heinemann and the other doctors in Tenma’s way are mysteriously murdered, and both children disappear from the hospital soon afterwards. The police suspect Tenma, since he benefits from the unfortunate turn of events; however, they have no evidence and can only question him.
Nine years later, Tenma is Chief of Surgery at Eisler Memorial. A patient of his, being questioned in connection with the murder of middle-aged couples across Germany, is murdered in front of his eyes, and the killer is revealed to be Johan Liebert, the child who disappeared. Tenma tells all to the police, but they find no trace of anyone named Johan Liebert, and Tenma soon becomes the lead suspect. Distraught that what he believed was the right thing has actually led to so much misery, Tenma begins to track down Johan, wanting to hunt the Monster down as atonement for his fatal mistake.