Just having exited a mysterious basement, former police office Hofmeister phones Inspector Lohmann to inform him that he has heard mention of someone working in the shadows…. The conversation is interrupted and Hofmeister suddenly starts singing a refrain. He has gone mad. Professor Baum, who heads the mental institution where Mabuse is housed, gives a lesson on the famed criminal, screening a documentary in which Mabuse is seen in his cell, afflicted by uncontrollable graphomania. The Inspector finds several letters traced on the window at Hofmeister’s house. Dr. Kramm, Baum’s assistant, tells the Professor of a sensational similarity between the writing of the patient and those found at the scene of a recent robbery. What if Mabuse were an impostor? Shortly thereafter Kramm is murdered by men from the basement gang. While the police decipher the markings on the window at Hofmeister’s house, which form the name Mabuse, the Inspector is informed that the criminal has died. Nonetheless, with the help of a gangster named Kent, member of the basement gang, Lohmann discovers that Mabuse is the leader of the gang and Baum works with him. A search of his house uncovers a record player transmitting his voice as well as a map of Berlin indicating the venue of a certain mission scheduled for that very evening: a pharmaceutical plant. By the time they reach it, the plant is in flames. Baum escapes in a car driven by the phantom Mabuse, releasing Lohmann and Kent. At the mental institution, Mabuse delivers his writings to the Professor and leads him to Hofmeister’s cell. Baum introduces himself as Mabuse. Hofmeister attacks him before the nurses intervene and, finally, the Inspector and Kent. The door of the cell closes and the camera focuses on Baum, gone mad.