This is a family story, told in turn by each of its members. The matriarch, Diana, a retired headmistress of repute, is suffering from Altzheimers. With recurrent episodes of sanity she still plays a pivotal role. Her sister-in-law, Aunt Betsie, is virtually responsible for bringing up the children and grandchildren, until, at the age of 39, she marries Dafydd, who is penniless, and has a learning disability. A most likeable fellow, he is blessed with a wonderful tenor voice. A few years after joining the family he inherits a fortune. With Betsie, Dafydd moves to the ancestral mansion in Wales, which they convert to a successful hotel. Diana's son, Adrian is a journalist, and her daughter, Harriet, a schoolteacher. Adrian marries Mildred, who plays the second pivotal role in the novel. They have two children, Sophie, an art student, who has a disastrous love affair with a tutor, and Nick, a fourteen-year-old boy who's frequently in trouble at school. Both their stories take up a considerable part of the novel, as does a love affair between Harriet and Adrian's black friend, Justin. Adrian embarks on an adulterous ill-fated liaison.
The major family crisis - the core of "Knock Knock" - occurs when Adrian has a stroke, from which he only partially recovers, and Mildred faces the impossible task of caring for both her husband and Diana, whose dementia has taken a steep downward turn. Dafydd, brain-damaged at birth, and a minor character in the story, plays a major role in the final pages.