London, the late 1970s. George Phillips, an estate agent working for the firm of Frobisher, Rendell and Ross, is asked whether he has found any tenants for Sunley House, a large office building in north London. Phillips gives various non-committal answers, but is goaded into action when ambitious young Rycroft asks some pointed questions and Welby, their boss, commends his initiative.

Phillips is clearly troubled by something: he barely interacts with either his colleagues or his secretary and shows little interest in joining in jargon-filled conversations. At home, he has a somewhat awkward family dinner, and is non-committal when his wife raises the subject of their son Robin sleeping with his girlfriend Jennifer in their house. Later, he shuts himself off by listening to opera on headphones while the others have a conversation.

Following a viewing with a group of clients, Phillips remains in Sunley House and explores the upper floors and becomes rather taken with the place. He finds out from the commissionaire that the building is unattended after hours.

He obtains the keys from Miss Morpeth back in the office, and is alarmed to be told by Welby that Rycroft has asked if he can try letting Sunley House himself. Mrs Phillips is staying with her ailing father, and Phillips finds it difficult to make small talk with Jennifer, who seems to have moved in permanently. That night, he slips out of the house and takes up residence on the top floor of Sunley House.

Miss Morpeth is interrogated by Rycroft as to the whereabouts of the Sunley House key. She can't find it, and he is annoyed by what he sees as the inefficiencies of her 'system' - writing down details in an old school exercise book. Phillips gets the key copied and surreptitiously returns it, but not before Rycroft has complained to Welby.

Phillips spends more and more time at Sunley House. Robin and Jennifer seem unperturbed by his absence, and move into his bedroom. Phillips neither knows nor cares, as he's happy in his own private world on the roof of the building, listening to music on his headphones.

He is rudely awakened when a security guard locks the hatch that he used to gain access, and finds that the only way he can escape is to use the window cleaner's lift to smash a window pain on the top floor - but this passes off without comment. Phillips is oddly exhilarated by this experience.

The next day, Rycroft is showing the property to American clients. While viewing the upper floor, he discovers Phillips' sunbed and cassette player. Realising that the building has an intruder, he lies in wait for his return - but Phillips is tipped off by the commissionaire by the entrance during some casual small talk. Although he fails to rise to the bait, he is convinced that his secret will soon be discovered and he returns to his office to clear his desk.

The next day, Welby and Phillips show their Japanese clients round the building. On the top floor, they discover Rycroft fast asleep, to Welby's horror. Later, Welby decides that Rycroft must be suffering from a mental problem and decides to let him go. Phillips returns home and, for the first time, refuses permission for Jennifer to stay the night.