April 1809, a stately home in Derbyshire... Thomasina, a gifted pupil, proposes a startling theory, beyond her comprehension. All around her, the adults, including her tutor Septimus, are preoccupied with secret desires, illicit passions and professional rivalries. Two hundred years later, academic adversaries Hannah and Bernard are piecing together puzzling clues, curiously recalling those events of 1809, in their quest for an increasingly elusive truth.