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 jailbreak works, but Gustav, the decoy, is seriously injured in the process when the prison van hits him. Tenma takes him to his former hospital, Eisler Memorial, and leaves Gustav to be collected by hospital staff. Eva Heinemann finally decides to testify in Tenma's defense until she hears about Tenma's escape. She also hears the voice of Roberto masquerading as Tenma's lawyer. After she flees to her hotel room, there is a knock on the door which she answers.
After hearing Roberto's threat against Eva, Tenma decides to confess to all charges against him so he can be placed in the same van transporting a prisoner named Gunther Milch to a Federal Penetentary as the prisoner is arranging for a jailbreak.
Former patients of Eisler Memorial Hospital enlist the aid of a lawyer named Alfred Baul to represent Tenma. Baul in turn asks for the assistance of Fritz Vardemann. Vardemann visits Tenma in prison, and agrees to take on the defense, but is then called to the hospital when his wife goes into labor with their first child. Following another drinking binge upon returning to Düsseldorf, Eva has a brief dream following her passing out recaping her near encounter with Johan Liebert the night Adolf Junkers was murdered.
Schuwald, with the help of Reichwein, re-explores his past with Karl Neumann's real mother and realizes his link to Johan. As news of Tenma's arrest spreads, various people (ranging from Dr. Reichwein, Hans Georg Schuwald, Dr. Schumann and Petra, Karl Neumann, Dr. Becker, former patients of Eisler Memorial Hospital, and the Vietnamese underworld doctor that Tenma once helped) react with shock and horror (with the exception of Eva Heinemann who is insistent that Tenma pay dearly for rejecting her).
Runge visits Suk in the hospital and tells him that the one truly responsible may be the last person he may suspect (a reference to Johan masquerading as Anna). Runge then confronts the guard outside Suk's room and demands to see Ranke. Runge reveals to Ranke that while he was able to obtain information on Emil Scherbe, Klaus Poppe, and Jacov Vyrobek (all the same person), he is not able to obtain information on Franz Bonaparta.