What links a prostitute, a Flemish painter and a tower watchman? All three claim to have seen a ghost on the same street in fifteenth-century Tallinn. And all of them have been found dead soon after. Melchior Wakenstede, apothecary, is charged with unearthing the truth behind these seemingly-supernatural murders. But when a pious and powerful merchant dies, along with a castrated and disfigured tramp, Melchior perceives a more medical connection between the deaths. Melchior must rise above the competing interests of merchant guilds, religious orders and Teutonic knights, to get to the bottom of the mystery.