In August 1939, thirty-year-old Hetty Cartwright arrives at Lockwood Manor to oversee a natural history museum collection whose contents have been taken out of London for safekeeping. She is unprepared for the scale of protecting her charges from party guests, wild animals, the elements, the tyrannical Major Lockwood, and Luftwaffe bombs. Most of all, she is unprepared for the beautiful and haunted Lucy Lockwood.

For Lucy, who has spent much of her life cloistered at Lockwood, suffering from bad nerves, the arrival of the museum brings with it new freedoms. But it also resurfaces memories of her late mother and nightmares in which Lucy roams Lockwood, hunting for something she has lost.

When the animals appear to move of their own accord and exhibits go missing, Hetty and Lucy begin to wonder what exactly it is that they might need protection from. And as the disasters mount, it is not only Hetty's future employment that is in danger but her own sanity. There's something, or someone, in the house. Someone stalking her through its darkened corridors . . .