A British official in Georgetown, British Guiana, after the death of "Mr. Abel," feels compelled to publish the story of his friend's adventures. He is, therefore, the narrator in the prologue of Green Mansions. The Englishman explains how he finally won Abel Guevez de Argensola's friendship so that the latter revealed the secret of his mysterious past, of the darkened chamber in his home, and of the urn containing ashes.
Throughout the twenty-two chapters of Green Mansions, Abel himself narrates his story chronologically, and the government official disappears completely from the plot. Abel was twenty-three years old when he was about to flee from Venezuela after the failure of a conspiracy to overthrow the government. Abel, implicated in the abortive coup, changed his mind about leaving the country and decided instead to fulfill a boyhood dream of exploring the untamed hinterlands south of the Orinoco river. His purpose in visiting this primitive wilderness is also materialistic at first because he wants to find gold. Suffering greatly from the hardships of the terrain, Abel is further discouraged by his realization that the natives have no golden objects in their possession and that he is unable to discover any gold deposits.
Reaching an Indian village in the Parahuari mountains, Abel is met by the natives and makes a favorable impression on Runi, the chief, by a gift of a tinder-box, his only remaining valuable trinket. Abel's welcome is then made more generous by the Indians invitation to join in drinking casserie, an alcoholic beverage. After this initiation, Abel is hospitably accepted by all the Parahuaris, especially Cla-cla, Runi's mother, and Kua-kó, a young brave of the tribe. Abel gradually accepts his new way of life and begins to appreciate the beauties of this wild paradise of nature.
After three weeks, Abel begins to explore the forests alone. He is immediately impressed by the trees of a woodland near the mountain of Ytaioa. The trees become for Abel the "green mansions," where he is very happy in the solitary natural surroundings. The Indians, however, are not pleased by Abel's report of his visit to the woodland; Runi, the chief, warns him not to go there again. Abel is surprised that they never enter that particular area to hunt or to roam. Fascinated by the unknown and mysterious, Abel eagerly pays more visits to the "green mansions" and is particularly delighted by the melody of a bird whose sound somewhat resembles a human voice.
Kua-kó warns Abel that an evil spirit dwells in the woodland, and the crafty savage begins to teach Abel the use of the zabatana, the weapon through which the Parahuaris blow poisoned arrows at game — and at their enemies. Abel, after a short time, becomes aware of two related factors shaping his future: An intelligent being lives in the woodland, and the Parahuaris want him to kill the "daughter of the Didi" with the blowpipe. During one of his frequent excursions to the forest, Abel is about to kill a snake when a beautiful young girl appears to protect the serpent. Abel is astonished to observe the manner in which the snake provides protection for the girl by curling itself around one of her ankles. However, Abel forgets the snake as he tries to approach the girl and is bitten by the serpent. Fearing that he will soon die, Abel runs toward the Indian village as a storm breaks. He loses his way, falls, and loses consciousness.
When he revives, Abel finds himself in the hut of Nuflo, an old man, who says that Rima, the bird-girl, is his granddaughter. The girl is shy indoors, and Abel cannot associate his previous ecstatic impression of her with the present surroundings. Although Rima is more talkative in the woodland, she continues to be shy with Abel as he endeavors to express his love to her. Nuflo and Abel, though they do not quarrel openly, nevertheless challenge each other in their constant discussions: The old man is reluctant to talk about his background and that of Rima, and Abel is constantly asking questions about the pair's past. Abel also discovers that Nuflo, contrary to Rima's wishes, kills animals in order to enjoy the taste of meat. Irked by Rima's avoidance of his presence, Abel leaves the woodland to spend a few days with the Parahuaris. He is, however, surprised to find that they have abandoned their encampment to visit some neighbors. Only Cla-cla, too feeble for the rigors of the trip, remains behind, and she and Abel enjoy an evening by the fire singing and talking.
Abel's anger at Rima's strange rebuffs disappears soon, and he hastens back to the "green mansions" to see Rima despite Cla-cla's pleas to stay at the Parahuari village. Caught again in a storm after going in the wrong direction, Abel is saved by Rima, who has been faithfully waiting in the woodland for him to return. Rima, though she is still unable to declare her obvious love for Abel, is more communicative with the young man, and they spend a day on top of Ytaioa mountain. Abel tries to tell the girl about the vast territories of the outside world, and he stresses the inner peace existing in the isolated "green mansions." Rima, however, is curious to know at first hand the sights beyond the woodland. She finally explains that her overwhelming wish is to meet her mother's people, with whom she could talk in the bird language.
Despite his effort to explain gently the truth about the probable annihilation of her tribe, Abel by chance mentions the mountains of Riolama, which Rima recognizes as the place where her mother belonged. She goes down Ytaioa quickly to upbraid Nuflo for concealing the location of Riolama and to make plans for a journey there. Abel sorrowfully follows Rima because he fears that the idyll of the "green mansions" is ending or, at least, is facing a crisis.
Nuflo is furious at Abel's mention of Riolama, but the old man, through fear of Rima's prayers about his future salvation, is forced to undertake the trip. In return for his part in the expedition, Abel demands that Nuflo tell the truth about his and Rima's background. Visiting the Parahuaris to divert any possible suspicion from the savages about the planned absence from the woodland, Abel is treated as a prisoner, and he has to escape from the village to rejoin his two companions.
During the trip to Riolama, Nuflo gradually reveals to Abel the story of his relationship with Rima. Nuflo, a member of a band of outlaws, by chance met an injured woman who was pregnant. He took her to the settlement of Voa where her daughter, Rima, was born. The child, like the mother, spoke a mysterious language of the birds which Nuflo was unable to understand. When the mother died, Nuflo took Rima to the "green mansions," and the girl, sickly because of the climate at Voa, became healthy as she lived close to nature. Rima, protecting the forest life, is hated by the Indians because she prevents them from killing the birds and animals.
When the party reaches Riolama, Abel tries to explain to Rima that the girl's tribe was probably killed by warring natives and that her mother was the only survivor. Rinia, overcome emotionally by the explanation of the tragedy, faints. She turns to Abel for love and protection when she revives. Instead of going back to the "green mansions" with Nuflo and Abel, the girl determines to return alone in order to prepare clothes for herself and a more suitable welcome for Abel. Stunned by this sudden decision, Abel reacts too slowly to stop the girl's departure.
Abel and Nuflo start back on the difficult trip, and they immediately suspect disaster when Nuflo's hut is found burned to the ground. Searching for Rima in the woodland, Abel is more apprehensive when one of the Indians, who have never dared to enter the "green mansions," appears and confidently leads him to the Parahuari village. After Runi berates Abel before the other savages, the young man explains convincingly that he had only left the encampment to search for gold. Abel eventually persuades Kua-kó to explain what happened to Rima. The Parahuaris, realizing that the woodland was no longer protected by Rima, began to hunt in the territory. One day they trapped the returning Rima in a tree. They built a fire around the tree, and Rima plunged to her death in the flames, calling to Abel as she fell.
Unable to sleep after hearing the tragic story, Abel tries to run away from the natives; but Kua-kó pursues him, and Abel kills the savage. Abel goes to the camp of Managa, Runi's enemy, and leads these Indians on a massacre of the Parahuaris. The sight of the dead Cla-cla revives Abel's better instincts, and he escapes to Nuflo's hideaway, where he finds the old man slain by the marauding savages. Abel endeavors to live alone, but he realizes that he will go insane because of the memories of Rima associated with the "green mansions." He searches for and finds the remains of Rima, determined to return to civilization where he can do silent homage to Rima for the rest of his life.
Although Abel is weak and the trip is long, he finally reaches Georgetown, where he recovers and establishes himself as a respectable figure, but one of mysterious, unexplained origins.