The story focuses on Scottish writer J. M. Barrie, his platonic relationship with Sylvia Llewelyn Davies, and his close friendship with her sons, who inspire the classic play Peter Pan, or The Boy Who Wouldn't Grow Up.
Following the dismal reception of his latest play, Little Mary, Barrie meets the widowed Sylvia and her four young sons in Kensington Gardens, and a strong friendship develops between them. He proves to be a great playmate and surrogate father figure for the boys, and their imaginative antics give him ideas which he incorporates into a play about boys who do not want to grow up, especially one named after troubled young Peter Llewelyn Davies. His wife Mary, who eventually divorces him, and Sylvia's mother Emma du Maurier, object to the amount of time Barrie spends with the Llewelyn Davies family. Emma also seeks to control her daughter and grandsons, especially as Sylvia becomes increasingly weak from an unidentified illness.