1809, a Regency room in Sidley Park. Thomasina Coverly, is a precocious teenager with ideas about mathematics well ahead of her time. She studies with her tutor Septimus Hodge, a friend of Lord Byron.
In the present, Hannah Jarvis, a writer, Valentine Coverly, a mathematician and Bernard Nightingale, an academic, converge on the house. As their investigations unfold, the truth about what happened in Thomasina's lifetime is gradually revealed.