It is 1618 in the German duchy of Württemberg. Plague is spreading, the Thirty Years' War has begun, and fear and suspicion are in the air throughout the Holy Roman Empire. In the small town of Leonberg, Katharina Kepler, an illiterate widow, is accused of being a witch. Katharina is known for her herbal remedies and the success of her children. Her eldest, Johannes, is the Imperial Mathematician and the renowned author of the laws of planetary motion. It's enough to make anyone envious, and Katharina has done herself no favours by going out and about and being in everyone's business. So when the deranged and insipid Ursula Reinbold (or as Katharina calls her, the Werewolf) accuses Katharina of making her ill by offering her a bitter, witchy drink, Katharina is in trouble. Her scientist son must turn his attention from the music of the spheres to the job of defending his mother. Facing the threat of financial ruin, torture and even execution, Katharina tells her side of the story to her friend and neighbour Simon, a reclusive widower imperiled by his own secrets.