1978. Marwood Manor is home to Lord and Lady Ashton, and their young son, Rupert, hidden from the world due to poor health. Living amongst them is their dutiful housekeeper, Mrs. Medlock, who has worked for them for over twenty years.

But in that time, Mrs. Medlock has become the keeper of secrets. Secrets of the family. Secrets of her own. Secrets that may be buried in the walled garden in the grounds of the manor, out of sight but not forgotten.

When a distant orphaned relative, Mary, arrives at the property in need of a place to live and with a determination to explore, Mrs. Medlock fears the secrets of Marwood are no longer safe. And digging up the past may have deadly consequences for them all . . .