Professor Jason Cavendish is the septuagenarian author of Cloud Cape, a children's fantasy novel. He lives in a secluded cliff-top mansion with his second wife Linda, his 18-year old daughter Christabel and Mr Hill, his butler and personal secretary. They are visited one day by Daniel Young, who claims to be writing a thesis on political allegory in children's literature. After saving Cavendish's life when the old man has a seizure, Daniel is invited to stay by Linda and Christabel who fight for his affections. Mr Hill, meanwhile, is suspicious of Daniel's motives and concerned by Cavendish's reluctance to show him what he is writing.
Daniel seduces Christabel and, unknown to the others, murders Linda. As an anxious Hill forces him to leave, the young man reveals his true name is Daniel Cartwright and that his father Andrew was a British intelligence officer who was murdered by Cavendish while escorting a Soviet defector to the British embassy. Cavendish leads Daniel to a summer house at the bottom of the garden where the author reveals he has been writing his memoirs, implicating himself and Mr Hill, as well as several high profile MPs, as Soviet sympathizers. Daniel convinces Cavendish to surrender the papers and shoot himself; the old man obliges, having grown weary of the enforced secrecy of his final years. Having discovered Linda's body, Hill arrives at the summer house to execute Daniel. The young man reveals that he has been sent by the KGB at Hill's request to prevent Cavendish blowing their cover, and that Linda was a sleeper agent for MI6. Daniel leaves Hill to clean up the mess and leaves. The remorseful Hill approaches the summer house to attend his beloved friend's body.