During a 3-days school trip to a medieval castle, Charlys' elder sister Nicole starts working as trainee-teaching monitor. Dandy is not supposed to come along, but Fred carries him so Charly uses gastroenteritis as a smokescreen to cover the dog. Once in the castle, the teachers fall ill one by one, with symptoms of poisoning. Charly makes a list of suspects ¿ but this time he has to include himself! Trouble is Mr. Siggi thinks Charly's the origin of the epidemic!