When the president of a chocolate factory, Ruggiero Onorati, dies from eating a poisoned chocolate, Terzani and Rex focus their investigation on the man's family, where various bitter conflicts come to light. The victim's sister, Giulia Onorati, had angrily fallen out with him over company strategy concerning the family chocolate business. However, the investigations are turned upside down when an epidemic of botulinus poisoning - a toxin contained in a batch of chocolate snack biscuits produced by the victim's company - hits Rome.

A female teacher taking part in a school outing to Rome comes to the police station to report that one of her students, Sara Borelli, has been bullied and drugged while she was in the hotel. Interrogated by Terzani, Sara recounts how she had been unwittingly drugged by classmate Clara Moretti. But then Clara's dead body is found just near the police station, having fallen from the bathroom window of her hotel room. What at first had seemed a tragic accident soon turns out to be a case of murder among classmates - and possibly a teacher. But the terrible truth will require all Rex's skills and intelligence before it is finally revealed.