Witchy Illusions recounts the trial of Mademoiselle Ambrosine, a girl of fifteen accused of witchcraft in France in autumn 1515. Her lawyer is Barthélemy de Chassenée, a historical figure who became famous when in 1508 he defended the rats accused of eating the people's grain stored at Autun's granary and growing in nearby fields of the département of Saône-et-Loire.

During Mademoiselle Ambrosine's trial, justice plays out erratically, and nothing is ever clear. The proceeding turns increasingly opaque and the issues become more convoluted and muddled by legal precedent. Arguments about God's will, mankind's place in nature, and whether demons defecate and have erections obscure focus on the central issue of the defendant's practice of witchcraft. These and similar metaphysical issues puzzle and invigorate everyone, the court and spectators alike, and it becomes evident that Inquisitor Institoris might have met his match in Mademoiselle Ambrosine.