Set in the late Middle Ages, "Zweikampf" follows the investigation into the murder of Duke Wilhelm von Breysach. The evidence points to his brother, Count Jakob the Redbeard, as the perpetrator. Jakob, however, denies the accusation and claims to have been with the virtuous Lady Littegarde von Auerstein on the night of the murder. As the investigation delves deeper into a world of political intrigue and personal vendettas, the truth behind Wilhelm's death becomes increasingly elusive.