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 Children from Petrov's former orphanage try to prove that Grimmer was not responsible for Petrov's death, but one of the children encounters Anna Liebert (who is actually Johan in disguise). When the child admits that he wants to find his mother, Anna sends him to a nearby red light district to show the child that he is not wanted. Grimmer and Tenma are able to intercept the child before he commits suicide.
Nina is confused by her recent memories. Once they confirm Suk's safety, Grimmer and Tenma reach an agreement with Ranke to obtain the tape and research materials and retrieve them from Suk's mother. However, they discover the research materials gone, and the tape has been recorded over. Inspector Runge decides to take a "holiday" in Prague, and meets up with his former colleague, Commissioner Nepela.
Nina arrives in Prague and senses something is wrong when the residents address her as "Anna Liebert". Tenma finds that Suk has been moved from the hospital to an undisclosed facility. Captain Karel Ranke, a former top member of the StB requests the presence of Tenma and Grimmer and requests an agreement to obtain the tape and research materials for an unknown party in Germany in return for Suk's safety. While they do not reach an agreement on this, Ranke does mention the name of Franz Bonaparta, who was heavily involved in the care of the Liebert Twins and was also a children's picture book author, and was living in a mansion covered in red roses.
Dr. Tenma learns about Suk's fugitive status from a journalist. And sets out to find him. Meanwhile, Suk and Grimmer goes into hiding, but is quickly found, surrounded and attacked by the STB. The STB's assualt comes to a bloody end.
Suk is put under surveillance by the new commissioner. Together with Grimmer, Suk retrieves the tape and research papers from the deposit box. But their movements is watched by various sides.