During a 3-days school trip to a medieval castle, Charly's elder sister Nicole starts working as a trainee-teaching monitor. Dandy is not supposed to come along, but Fred carries him so Charly fakes an illness 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!