Thomas Edgar, an amateur butterfly collector, comes back traumatized from an expedition to the Amazon and his young wife tries to find the cause. Thomas Edgar (27), an amateur naturalist in Richmond, England, joins an expedition to the Amazon to collect plant and animal specimens under the patronage of Senhor Santos, a Portuguese rubber tycoon in Brazil. Thomas dreams of finding a butterfly that has never been collected and wants to name it Papilio sophia, after his wife Sophie, whom he leaves behind in England.
The other three naturalists of the expedition are John Gitchens, a botanist, George Sebel, an entomologist, and Ernie Harris, a medical doctor, taxidermist and ornithologist.
They arrive in Belem, Brazil, in October 1903, where they stay for two months, going on daily outings into the forest to collect specimens. Thomas is led astray by Ernie and exposed to strong coffee, alcohol and smoking. His naturally nervous state is not improved by these addictions. As an amateur he also feels inferior to the other educated men.
In December the men, accompanied by Santos' man Antonio, follow the Amazon River west, as far as Santarém. They then row further upriver with the Rio Tapajós, a tributary of the Amazon. On the way they visit Captain Arturo, who gets very upset when he hears who their benefactor is. They stay at a rudimentary camp for two weeks, continuing their excursions into the forest. Here they are visited by José, a hat merchant from São Paolo, with his servant Manuel, who has lost his tongue in an “accident”. They talk about Santos and mention the incident with Captain Arturo. Later they hear that Arturo's house was burnt down with him inside.
In May 1904 Sophie receives an unexpected note from Mr. Ridewell to say that Thomas will be arriving back that week. He is much changed – thin, dishevelled and full of non-healing sores. He does not speak or try to communicate with anyone.
In January 1904 the expedition travels further up the Amazon to Manaus, where they finally meet Santos – he is the “hat merchant” they met earlier. He invites them to dinner at the club he owns, where there is gambling, drinking, smoking and women. He offers Thomas a cigarette which secretly contains a hallucinatory drug.
Thomas starts feeling very strange and stumbles outside. He comes upon a street party and has hallucinations about his butterfly. He follows the “butterfly” and has intercourse with it, only to find out that it was a woman. The next day, when they are introduced to Santos' wife Clara, he realises that she was the butterfly woman.
The group, which now includes Santos and Clara, goes on an expedition up the Rio Negro. Thomas tries to avoid Clara, but when Santos goes back to Manaus for a few days, she visits Thomas in his hut. She confesses that Santos is not interested in her. She offers Thomas drugs. They make love.
Now Thomas, feeling extremely guilty, tries to avoid Clara even more. He ventures into the jungle on his own and comes upon a whole swarm of his special butterflies. He catches one, but cannot bring himself to kill it. He falls asleep. When he wakes up, the butterflies are gone. He stumbles back in a fever – he has malaria and isn't sure whether the butterflies were once again only a dream.
In Richmond, Captain Samuel Fale had wooed Sophie while Thomas was away. When he realises the state that Thomas is now in, he plans to get him declared insane, with the help of Sophie's father, Mr. Winterstone. Sophie, determined to find the cause of Thomas' illness, reads his journal and finds out about Clara. She is angry and nearly throws herself on the mercy of Captain Fale.
On the recommendations of her friend Agatha Dunne (who is having an affair with the widower Robert Chapman), Sophie takes Thomas to see his entomologist friend Peter Crawley at Kew Gardens. In the rainforest enclosure they talk about rubber plants and Santos, which upsets Thomas and he runs off. He returns home drenched. As Sophie undresses him, her desire flares. That night she climbs naked into his bed and they make love.
The next day Thomas ignores her again. In anger she drags his butterfly cases outside and starts to burn them. This finally snaps him out of his illness and muteness and he tells her all.
In March 1904 Thomas, still weak from his illness, leaves his friends behind and returns with Antonio to Santos' house in Manaus. He hears about the atrocities happening on Santos' rubber plantations. He talks to Benedito Rodrigues, whose printing press Santos has burnt down. The next day he finds Rodrigues tortured and murdered.
Thomas hastens back to the expedition to warn his friends of Santos' real nature. Santos has contracted syphilis from the Manaus prostitutes. He publicly accuses Clara of having affairs with the four Englishmen. When George (whose secret homosexuality Santos has discovered), Ernie (whose prostitute is paid for by Santos) and John (who is truly in love with Clara) falsely admit to having had sex with her, Santos kills her. John drowns himself and Thomas is sent back to England, unable to safely expose Santos' deeds. He never finds his special butterfly.
Eight years later Thomas and Sophie have children and he is now a professional collector. Agatha has married her lover. Santos has been exposed, but died before he could stand trial. Ernie and George died in Brazil from unknown causes.
Best part of story, including ending:
The suspense of not knowing what happened in the jungle to totally traumatize Thomas
Best scene in story:
Thomas, half-delirious with malaria, rushes into the jungle on his own and thinks himself in the midst of a swarm of his special butterflies.
Opinion about the main character:
Thomas does not stick to his principals and too easily gives in to temptations (smoking, drink and sex).