Elderly professor Jason Cavendish collapses while sitting out in the sun. A stranger arrives and quickly revives him. Jason's daughter Christabel and her stepmother Linda are initially wary of the young man, who later identifies himself as Daniel Young, a student preparing a thesis on one of Cavendish's books. When Jack Hill, the Cavendishes' butler and general handyman, hears that there is an unexpected visitor, he quickly gets a gun before serving refreshments.

Daniel is invited to stay for dinner and then takes Christabel for a walk while Jason rests. Jack wakes Jason from his afternoon nap and tells him that he is suspicious of the young man. Daniel and Christabel are seen kissing by Linda, who seems jealous of them. At dinner, Daniel asks if Jason knew the Cambridge spies Philby, Burgess and Maclean when they were students. Jason says that he didn't, but is clearly upset by the question.

Christabel and Daniel sleep together, but afterwards he insults her and leaves. When Jack finds Jason still awake, he asks him about the manuscript he is working on, which he keeps hidden. Jason refuses to tell him and so Jack refuses to let him have a drink.

In the morning, Daniel and Jason talk while watching Linda and Christabel play tennis. Jason asks if he has any specific questions to ask for his thesis, but Daniel says that he already has all the information he needs. Daniel follows Linda into her shower and strangles her.

Jack makes a phone call, using the password 'Blade on the feather', requesting help to stop Jason writing his book. During tea, Daniel takes Jack's gun and demands to know where Jason has hidden his memoirs. He reveals that his real name is Cartwright and that he is the son of a man Jason had killed to prevent himself from being exposed as a KGB agent. Jack locks Christabel in her room and when he goes looking for Linda finds her dead body.

At gunpoint, Jason leads Daniel to the summerhouse and gives him the manuscript. He tries to justify his treachery and admits to being a complete physical coward. Daniel makes Jason shoot himself and then tells Jack that he is in fact the man sent by their shared KGB employers to stop Jason from writing his memoirs. He killed Linda because she was an MI5 agent.