In a room locked from the inside, the body of the Cavalier de Meve was found with nine stab wounds. His former maid Agatha Prejean is accused of murder, threatening to take revenge on him for his insult, even if she has to sell her soul to the devil. During interrogation, the woman admitted that she killed de Meve using witchcraft, for which she was put on trial. However, King Joan intervenes in the case, and considers the accusation unproven. He orders the secret police to re-investigate. Having taken it up, Baron de Segur unexpectedly learns that visiting merchants in Saint-Marco are disappearing without a trace, and someone is selling their stolen goods wholesale to local merchants. Now Mark has to not only solve the murder of the Cavalier de Meve, but also catch those who kill and rob merchants in the capital of the kingdom.