Hubert and Girwidz ring the doorbell of Arno Heinrich, who had reported his wallet stolen. It is the fifth wallet theft in a short period of time, all occurring in the vicinity of the Rattlinger bakery. Inside, the officers find the homeowner dead on the couch, with a whiskey glass containing the remnants of a pungent liquid in front of him.

The young Janina Schulz lives with Arno Heinrich. He was her professor and the most famous philosopher in Germany—suicide is out of the question for her. Dr. Fuchs soon confirms this, as no fingerprints can be found on the glass containing the lethal substance. The poison used is the same that once killed the philosopher Socrates.

The victim's calendar quickly leads Hubert and Girwidz to the reclusive Professor Reinfried Gluck, Heinrich's superior. However, all they get from him for the moment is a severe irritation of Girwidz's nasal passages after an overdose of snuff. Through Janina's mother—who is trying to mend the strained relationship with her daughter—the investigators become aware of Janina's ex-boyfriend Markus.

But here, too, Hubert and Girwidz seem to have reached a dead end. The young lawyer vehemently insists on his innocence. Only when Janina discovers mysterious threatening letters in Arno Heinrich's desk and a break-in occurs at the victim's house do the investigations gain new momentum…