In the summer of 1350, as a deadly pandemic tore across Europe, nine people were burnt at the stake in the Hanseatic city of Visby on the island of Gotland. Ostensibly, this was a sadly familiar story: the Black Death was rampant, and a supposed Jewish plot was offered as an explanation. The alleged perpetrators were forced to confess, and were duly executed. But the people who were consigned to the flames in Visby were not Jewish. In fact, there were no Jews on the island to be victimised. Of the nine accused, two were Christian preachers, and the only named individual, Tidericus, was an organista -- probably the organ player at the church of St. Olaf in Visby. The outlandish theory was that the plague was a poisoning scheme run by an international conspiracy of Jews, oligarchs and secular administrators. This book traces the story of the unfortunate Tidericus, and examines the implications of the antisemitic fantasy which led to his death. The positions of the different ethnic groups in Visby is considered against the backdrop of panic and paranoia which pandemics inspire. The interests of the native Old Gutnish-speaking population are compared to the motives of the Low German-speaking mercantile elite who were administering Visby at the time. A context of social unrest, with class divisions bisecting proto-national identities, proved to be a dangerous fuel for a surreal conspiracy theory. A popular willingness to believe in this lurid nightmare proved to be the undoing of Tidericus and his co-accused. The sources which record the affair, namely two pieces of Hanseatic correspondence, are edited and translated into English for the first time in the appendices.