Tony Last is a country gentleman, living with his wife Brenda and his eight-year-old son John Andrew in his ancestral home, Hetton Abbey. The house is a Victorian reconstruction in ersatz Gothic style, described in local guide books as devoid of architectural interest but is nevertheless Tony's pride and joy. Entirely content with the simple country life, he is seemingly unaware of Brenda's increasing boredom and dissatisfaction, and of his son's developing waywardness. When Brenda meets John Beaver, a social parasite controlled by his unscrupulous property-developing mother, she is quickly captivated by his superficial metropolitan gloss, and the two begin an affair. Brenda starts spending her weeks in London, and persuades Tony to finance a small flat, which she rents from Mrs Beaver. Although the Brenda–Beaver liaison is well known among London's society crowd, Tony remains oblivious; attempts by Brenda's friends to compromise him into a relationship with the glamorous and promiscuous Jenny Abdul Akhbar are unsuccessful.
Brenda is in London when John Andrew is killed in a riding accident. On being told that "John is dead", Brenda at first thinks that Beaver has died; on learning that it is her son John, she betrays her true feelings by uttering an involuntary "Thank God!". Thereafter she shows little emotion, and when the funeral is over tells Tony that she wants a divorce so that she can marry Beaver. On learning the extent of her deception Tony is shattered, but agrees to protect Brenda's social reputation by allowing her to divorce him, and to provide her with £500 a year. After spending an awkward weekend in Brighton contriving divorce evidence, Tony learns that, pressed by Beaver and his mother, Brenda is now demanding £2,000 a year—a sum that would require Tony to give up Hetton. This is too much for him, and his disillusionment with her is complete. He withdraws from the divorce arrangements, and announces that he intends to travel for six months. On his return, he says, Brenda may have her divorce, but without any financial settlement.
With no prospect of riches, Beaver loses interest in Brenda, who is thus left adrift and in poverty. Meanwhile, Tony has met an explorer, Dr Messinger, and joins him on an expedition in search of a supposed lost city in the Brazilian forest. On the outward journey, Tony engages in a frustrated shipboard romance with Thérèse de Vitré, a young girl whose strict Roman Catholicism causes her to shun him when he tells her he has a wife. In Brazil, Messinger proves an incompetent organiser; he cannot control the native guides, who abandon him and Tony in the depths of the jungle. Tony falls ill, and Messinger takes off in their canoe to find help, but is swept over a waterfall and drowned.
Tony wanders in a delirium until he is rescued by Mr Todd, an ancient settler who rules over a small community in an inaccessible part of the jungle. Todd is at first solicitous towards Tony, and nurses him back to health. Although illiterate, Todd owns copies of the complete novels of Charles Dickens, and Tony offers to read to him. However, when Tony is recovered in health and asks to be helped on his way, the old man repeatedly demurs. The readings continue, but the atmosphere becomes increasingly menacing as Tony realises he is being held against his will. When a search party finally reaches the settlement, Todd arranges that Tony be drugged and kept hidden; he tells the party that Tony has died, and gives them his watch to take home. When Tony awakes he learns that his hopes of rescue have gone, and that he is condemned to read Dickens to his captor indefinitely. Back in England, Tony's death is accepted; Hetton passes to his cousins, who erect a memorial to his memory, while Brenda resolves her situation by marrying Tony's friend Jock Grant-Menzies.