A film that felt surprisingly modern, though for once it's not really meant as a 100% compliment. Mabuse isn't unlike a bunch of police thrillers from the 90s, with the cops chasing the case of a mysterious serial killer. I think I would've preferred a full-on German Expressionist film, but with the introduction of sound that era was clearly dying.
Mabuse is a master criminal. He is locked up in an institution, though his doctors were never able to figure out what exactly went on inside is head. When the police are confronted with a series of murders that happened exactly as Mabuse wrote them down, they are stumped and start an investigation.
Lang didn't forget his learnings from 20s silent cinema, but the introduction of sound puts more focus on the narrative, which ultimately made it a more tedious affair, certainly at 120 minutes. The case is quite interesting and the pacing is decent, it's just that the execution is rather basic. But if you like these kinds of police thrillers, then this is well ahead of its time.