Monster is an mystery story, set in Germany, about a Japanese surgeon named Doctor Kenzou Tenma, trying to undo a big mistake, and prove his innocence. Years after performing a life-saving operation on a young boy instead of the mayor, Tenma soon discovers that the boy he saved, Johan, is a manipulative psychotic killer who will stop at nothing to create the world he envisions in his twisted mind. Soon, Doctor Tenma finds himself the prime suspect for the numerous murders Johan has committed.
The murder of Commissioner Hamrlik and the two corrupt officers stirs up a media frenzy. Suk questions the kids of Petrov's orphanage to find any leads. Tenma arrives in Prague to look for information regarding the mother of the Liebert Twins. Suk tries to get close to the Nina Fortner lookalike who calls herself Anna Liebert. Anna Liebert is later revealed to be Johan masquerading as Nina.
Zeman's protege, Detective Jan Suk, investigates Filip Zeman's murder and uncovers Filip Zeman's link to the StB and two corrupt officers on the force. He reports this to his commissioner Hamrlik, not knowing that Hamrlik is in league with the corrupt officers. Hamrlik and the two officers Chief Detective Batella and Detective Janacek are later poisoned by candy. While this is happening, Zuk confides to a girl who appears to be Nina Fortner.
Inspector Filip Zeman of the Prague police meets with Grimmer and questions him about the death of Petrov. However, Filip Zeman reveals himself to be working for former members of the Czechoslovakian StB and tortures Grimmer for information. The woman who resembles Nina interrupts the interrogation by shooting one of the StB members dead. Grimmer wakes up and notices that everyone else has been beaten to death and concludes "The Magnificent Steiner" is responsible.
Grimmer encounters Petrov, who worked as a psychiatrist at 511 Kinderheim and now is the headmaster of an orphanage. After playing with the kids from Petrov's orphanage, they return and notice a woman who resembles Nina leaving the orphanage. They also find Petrov mortally wounded and his assistant dead. Petrov gives Grimmer a key to a safety deposit box which contains evidence of Johan's origins before he dies.
Wolfgang Grimmer is a freelance journalist trying to investigate allegations of child abuse and psychological reprogramming in the former East Germany orphanages. While on a train to Prague, he shares a booth with Dr. Tenma, and assists in Tenma's escape when guards arrive to arrest him.