For the return of the classes, Mélisande has a brilliant idea: to register in the school of Mélusine! Very quickly, Mélusine realizes that her fairy cousin will not hold long in the dreadful world of sorcerers where trepanation rhymes with vivisection and where the teachers spend their time learning the art of throwing terrible evils or that of inventing terrible potions. But Mélisande does not have time to be disgusted for ever from the teaching that another event comes to disturb the school: more and more students fall ill. Would they be victims of a drug? But who might have better to harm this noble institution? The investigation is open and it is a real inquisition that deals with the school and the two cousins...