Tony Last is a country gentleman living with his wife, Brenda, and their eight-year-old son, John Andrew, at Hetton Abbey, their ancestral home. Hetton house is a Victorian neo-Gothic pastiche described as architecturally devoid of interest, by a local guide book, and ugly, by Brenda, yet hetton Abbey is Tony's pride and joy. Content with country life, Tony is unaware of Brenda's boredom and dissatisfaction and of John Andrew's waywardness. In the event, Brenda meets John Beaver, and, despite acknowledging that he is a dull and insignificant man, she begins a sexual affair with him. She then starts spending her weeks in London, and persuades Tony to finance a small flat, which she rents from Mrs. Beaver, John's mother, who is a canny businesswoman. Although the affair between Brenda and John is known to their social circle in London, yet Tony remains oblivious; attempts by Brenda and her friends to set him up with a mistress are absurdly unsuccessful.
Brenda is in London when their son is killed in a riding accident. On being told that "John is dead", Brenda at first thinks that it is Beaver who has died; on learning that the dead John is her son John Andrew, Brenda betrays her true feelings by uttering an involuntary "Thank God!" After the funeral, she tells Tony she wants a divorce so she can marry Beaver. 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 with Milli — a woman hired to contrive divorce evidence — Tony learns from Brenda's brother that, encouraged by Beaver, Brenda is now demanding £2,000 a year: a sum that would require Tony to sell Hetton. Tony's illusions are shattered. Luckily, because Milli brought her child with her to Brighton Tony is able to establish he did not commit adultery. He withdraws from the divorce negotiations, 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 Tony's money, Beaver loses interest in Brenda. Meanwhile, Tony has met the explorer, Dr Messinger, and joins him on an expedition in search of a supposed lost city in the Amazon rainforest. On the outward journey, Tony has a shipboard romance with Thérèse de Vitré, a young girl whose Roman Catholicism causes her to shun him when he admits he is married. In Brazil, Messinger proves an incompetent organiser; he cannot control the native guides, who abandon him and Tony in the depths of the jungle. They attempt to trek out, but Tony becomes too ill to walk. Messinger leaves in their only canoe to find help, but is swept over a waterfall and killed.
In delirium, Tony wanders until he is rescued by Mr. Todd, a British Guianan, who rules a small, extended family in a clearing in the jungle, where Todd nurses Tony back to health. Although illiterate, Todd owns copies of the complete works of Charles Dickens and asks Tony to read to him. When Tony's health recovers and he asks to be helped on his way, and old man Todd repeatedly demurs. Tony continues with the readings, but the atmosphere becomes menacing, as Tony understands that he is being held against his will. When a search party of Europeans approaches the jungle settlement, Todd sedates Tony, keeps him hidden, and gives the search party Tony's wristwatch because "they wanted something to take back to England, where a reward is being offered for news of [Tony]". When Tony awakens he learns that his hopes of rescue are gone, and that he is condemned to indefinitely read Dickens to his captor in the Brazilian jungle. Back in England, Tony's social circle accept his death; Tony's cousins inherit Hetton Abbey, who erect a memorial to Tony; and Brenda marries a friend of Tony's.
