December 1840. Surgeon turned piano-player Benjamin January is looking forward to a peaceful holiday with his family. But the arrival of an old friend brings unexpected news—and unexpected danger.

Persephone Jondrette has found Arithmus: a Sudanese man with extraordinary mental abilities who January last saw in France, nearly fifteen years ago, during a ghost-hunting expedition to a haunted chateau. January and his friends survived the experience . . . but Arithmus's benefactor, the British explorer Deverel Wishart, did not. He was discovered dead one morning, his face twisted in horror, and shortly afterwards Arithmus vanished, never to be seen again.

Did Deverel succumb to the chateau's ghosts—or did Arithmus murder him and run away? January is determined to uncover the truth about the tragic incident from his past, and clear his old friend's name—but even he isn't prepared for what happens next . . .