In 1964 England, Helen Hansford works as an art therapist at Westbury Park psychiatric hospital near Croydon. She is three years into a secret affair with Dr. Gil Rudden, a married psychiatrist at the same institution whose wife, Kath, is Helen's distant cousin by marriage. Gil has promised Helen she is the love of his life but refuses to leave Kath until both their children finish school, the youngest being only seven. The affair demands constant secrecy and has cost Helen all close friendships.
Gil cancels a promised weekend together because Kath has crashed the car, but a phone call from Gil the next day brings news: Police have discovered a man who has not seen daylight in at least a decade, living in squalor with his elderly aunt in a house near Lloyd Park. At the crumbling villa, Helen meets 74-year-old Louisa Tapping, who is frantic about her nephew, William. Gil persuades the silent William Tapping, 37, to come downstairs voluntarily. William emerges wrapped in a tartan blanket, his face obscured by matted hair and a long beard.
Gil admits William to Westbury Park and designates him a garden patient, granting him unsupervised access to the hospital grounds. Helen interviews Louisa, learning that William last went outside in 1944 and can speak but chooses not to. When Louisa dies of pneumonia shortly after, Helen retrieves a hidden tin box containing obsolete banknotes, three unopened letters addressed to William in a child's handwriting, and a newspaper obituary of a painter named Douglas Samsbury. In William's attic room, Helen discovers over a hundred ink drawings of extraordinary quality and repeated sketches of a cottage in a garden. She takes several drawings and his fountain pen, recognizing a unique opportunity to reach him.
Chapters set in earlier decades reveal the forces that shaped William's captivity. William is the illegitimate son of Rose Tapping and Douglas Samsbury, a painter. The family arranged for him to be adopted by Rose's brother and his wife, who died in a car crash when William was six weeks old. Rose brought him home as her "nephew," never able to acknowledge him publicly. Three aunts raised him: the stern Elsie, the anxious Louisa, and Rose herself, the only one who recognized the injustice of his confinement.
At eleven, William attended Belwortham Hall boarding school, where he befriended a boy named Francis Kenley. During a summer holiday at Brock Cottage, the Kenleys' home in Sussex, William discovered what family life could be. Francis's mother, Marion Kenley, played cricket, Mr. Kenley read aloud by candlelight and taught William to float in the sea, and the family woke the boys at midnight to watch badgers. William treasured a silver badger napkin ring, a talisman he kept for the rest of his life.
Back at school, a teacher named Mr. Swales began summoning Francis to private sessions. William followed Francis to a forbidden upper floor and discovered Swales sexually abusing him. In shock, William shoved Swales, who fell through an open trapdoor and broke his neck. The boys called the Kenleys from a train station. Francis's father, a barrister, pressured the headmaster into calling the death accidental and keeping the boys uninvolved. Aunt Elsie, however, withdrew William from school and hid him at home permanently, omitting him from wartime registration forms to conceal his existence. Over the decades, the aunts' protective instincts calcified into imprisonment. William was never told no one was looking for him. Letters from the Kenleys were intercepted and hidden, and Marion later admitted she failed to follow up because the Kenleys, too, wanted to forget.
Rose tried to marry a colleague and take William with her, but Elsie forbade it. In 1944, Rose was killed by a V-1 rocket, a German flying bomb, after following William outside. Elsie died in 1956, leaving the increasingly confused Louisa alone with William until a quarrel over a dead magpie alerted neighbors who never knew a man lived in the house.
Back in 1964, Helen returns William's pen and drawings and invites him to the art room. His lips move silently to form "thank you," his first attempt at communication. He begins visiting daily, producing free, flowing sketches entirely different from his meticulous ink work. Helen tracks down Francis and Marion through a former schoolmate who recognized the newspaper story. Francis is reluctant to visit, but Marion is eager.
The Kenleys' visit produces a breakthrough. William speaks for the first time, in a high, hoarse, childlike voice, saying Francis's name and asking, "Why do you look so old?" (197). He tries to return the silver napkin ring, believing he took it by mistake as a child. Marion tells him to keep it, and he accepts Francis's old letters with wonder rather than bitterness.
Helen's personal life unravels in parallel. She learns from her parents that Kath is pregnant, a fact Gil concealed. Helen articulates her impossible position: She cannot ask him to leave a pregnant wife without losing respect for him, nor continue the affair without losing respect for herself. Meanwhile, her niece Lorraine, a sixteen-year-old whose artistic ambitions Helen has encouraged, is admitted to Westbury Park after a mental health crisis during an exam. Gil agrees to speak with Lorraine unofficially, but the arrangement backfires: Lorraine develops an infatuation with Gil, who encourages it with compliments and gifts, including expensive shoes. When Helen confronts Gil in the art room, he blurts out "Oh, to hell with Lorraine!" (332), not realizing Lorraine can hear from the storeroom. Lorraine emerges devastated. William, arriving at that moment, sees Gil restraining Lorraine and drags him away. Gil strikes his head on a kiln, and William flees in terror, believing he has killed again.
Helen manages the aftermath. When she contacts Gil's wife about the injury, Kath reveals Gil was delighted about the pregnancy, contradicting his claims to Helen. Helen confesses to Dr. Lionel Frant, Gil's colleague, who responds with a professionalism she had not expected from him. Helen finds William at his old house, terrified, and reassures him Gil is alive. William shares a letter written by Rose and held by the family solicitor since 1937, revealing that Rose was his mother and Samsbury his father. Lorraine returns home and begins rebuilding her life, with Helen helping her explore art courses at Croydon Tech.
Gil resigns from Westbury Park after Lionel is appointed the new superintendent. He plans to join R. D. Laing's experimental community at Kingsley Hall, where doctors and patients live together without drugs or hierarchies. In a final conversation, Helen tells Gil she wishes they could have remained colleagues and friends. He replies that he could never have been just her friend.
Marion offers William a permanent home at Brock Cottage. Helen visits in September to find him flourishing: He walks dogs along country lanes, tends rose beds, and plans to build a henhouse. Marion has arranged for Francis to visit, mentioning Helen's presence to see if it would make a difference. The sound of his car confirms it has. In a final chapter, William dries the dishes, says goodnight to his father's painting and his mother's photograph, and stands at his bedroom window watching for badgers. Present happiness has replaced the old grief.