January 1917, Cornwall. Fifteen-year-old Simon Lysaght is sent, after the death of his father in France, to live with family at their estate house, Trevelyan Priors. His uncle, Sir David Trevelyan, is somewhat intimidating and Simon feels ill at ease in the large and forbidding house. On his first day Simon learns of the death of the eldest Trevelyan son in the war and the subsequent suicide of his fiancée, is warned never to wander the corridors of the house after dark by the younger Trevelyan son, crippled and bed-ridden William, and hears that a maid has jumped from a high window, killing herself. It is not an auspicious start to his life under the Trevelyan's roof.
Soon, however, his cousin Tom takes him under his wing and shows Simon the grounds and estate, including the still-functioning shot tower, a tall building which dominates the skyline. Exploring further one day after a heavy snow, the boys enter an enclosed garden, long abandoned and overgrown with dark avenues of trees and bushes and a formal fountain laid out in the centre. In the untouched snow, a set of footprints, with no discernible beginning or end. Who do they belong to, and how did they get there? Simon can see no answer, but is soon occupied by other, more urgent matters and the incident is pushed from his mind.
Weeks later, however, he has reason to recall the strange sight when he encounters a girl of his own age in the abandoned garden. She tells him her name is Lily, a fisherman's daughter from nearby Porthmullion. They strike up a friendship, although for reasons he cannot fully explain to himself, Simon keeps his new acquaintance a secret. He is soon to find that Trevelyan Priors and its inhabitants all have much to hide and more to tell than he can yet guess at...