The Testament of Dr. Mabuse ending explained

What happens

Lohmann stops the chemical plant plot and learns the voice behind the curtain was Professor Baum, who has been taking dictation from Mabuse's ghost. Baum collapses into madness and is locked in Mabuse's old cell.

The last image

After Baum is confined, Lohmann remarks that the idea Mabuse spread cannot be locked away, and the film closes on that note of unease.

What it says

The film ends this way because the plot was never the point. Mabuse is dead, yet his plan still runs through other people. Catching Baum removes a man, not the idea, and Lohmann knows it. That lingering fear is the warning.