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.