John Wiltshire, a British trader in copra, dried coconut meat, and the story's narrator, arrives by schooner at Falesá, a remote South Sea island village, to take over a trading station. The captain describes the settlement's troubled history: The previous trader, John Adams, died suddenly after becoming delirious, and the one before him, Vigours, fled in fear of the white men on the beach. These men are Case, a shrewd, charming trader of unknown nationality; Black Jack, a Black man; and Whistling Jimmie, who has since drowned. Case comes aboard and immediately impresses the lonely Wiltshire with his intelligence and easy manners. Wiltshire learns that his firm's previous representative abandoned the station in fear, leaving it unattended. Over drinks, Case assures Wiltshire that Falesá is safe and persuades him to leave his goods unguarded and come to Case's house that evening.
As they walk through the village, Case suggests Wiltshire needs a wife and points out Uma, a slender young island woman with a shy, striking appearance. Wiltshire agrees. Case brings Wiltshire to the house he shares with Captain Randall, a decrepit, gin-soaked old man who nominally owns the local trading operation but is exploited by Case and Black Jack. Randall reacts with alarm at the mention of Uma. That evening, Black Jack performs a sham marriage ceremony using a novel as a prayer book. The certificate, written by Case, declares that Uma "is illegally married to Mr. John Wiltshire for one week" and that he may discard her whenever he wishes (12). Wiltshire feels ashamed but accepts the deception, a common practice among traders in the region.
Uma leads Wiltshire to his station, running ahead joyfully; Case had told her Wiltshire was desperate to have her, and she believed every word. Moved by her dignity and tenderness, Wiltshire dumps his only case of gin off the verandah, partly for Uma's sake and partly from horror at Randall's example. Uma, deeply grateful, declares her total devotion.
The next morning, villagers gather silently across the river, staring at Wiltshire's house but refusing to approach. Not a single customer visits despite his impressive store. On the third day, Father Galuchet, a French Catholic priest, tries urgently to warn him. Uma confirms what the villagers already know: Case poisoned Adams with "white sand." Wiltshire visits Case, who deflects the accusation by blaming it on a personal grudge the priest holds. On Sunday, the native pastor spots Wiltshire near the church and stares at him in horror. By Monday night, Wiltshire concludes he has been tabooed, placed under a supernatural prohibition preventing islanders from trading with or associating with him. He confronts Case, who pledges to stand by Wiltshire as a matter of the "White Man's Quarrel," the principle that whites should support one another against islanders.
They appear before village chiefs, but the meeting yields nothing useful. The chiefs press Case with pointed questions that make him sweat visibly. Afterward, Case tells Wiltshire the villagers are simply frightened of him. The two part angrily, and Wiltshire realizes Case has maneuvered to monopolize all trade. Back home, Uma reveals the truth: the taboo belongs to her, not to Wiltshire. She offers to leave so he can trade freely, but Wiltshire, realizing he loves her, refuses. Uma explains that a young chief named Ioane wanted to marry her but suddenly withdrew months earlier, after which she and her mother, Fa'avao, were completely ostracized. Case, who had been secretly courting Uma, was the only person who still visited. Wiltshire realizes Case engineered Uma's isolation first to claim her, then to destroy Wiltshire's trade.
When a missionary boat arrives, Wiltshire intercepts Case along the way to the river and punches him to the ground. He introduces himself to Mr. Tarleton, the Protestant missionary, tears up the fraudulent marriage certificate, and asks Tarleton to perform a proper, binding marriage to Uma. Tarleton does so gladly. Wiltshire then tells Tarleton his full story and seeks help understanding Case's schemes. Tarleton reveals that Case controls the village through Namu, the native pastor, who has fallen into debt to Case and does his bidding. Tarleton discloses that Underhill, Adams's predecessor, was struck with paralysis; Case had the still-living man buried alive, with Namu praying at the grave. Case also undermined Tarleton's credibility by performing a conjuring trick implying the missionary's real motive was money. Tarleton warns that Case will certainly try to eliminate Wiltshire.
A month passes. Wiltshire, Uma, and Fa'avao harvest copra, dried coconut meat, from their own trees, but the commercial boycott holds. During excursions into the bush, Wiltshire learns that the uninhabited eastern end of the island is believed full of devils. The natives call Case
Tiapolo, meaning the great chief devil, as opposed to lesser bush spirits called
aitu, and believe he draws supernatural power from the spirit world. Wiltshire spots Case emerging from the haunted forest and resolves to investigate. He enters the dense eastern bush and discovers the source of Case's power: wind-powered harps that produce eerie singing, carved figures with shell eyes and real hair, and a cellar containing a large carved face coated in luminous paint that glows in the dark. Together, these form a convincing devil-temple designed to terrify any native brought there at night.
Returning to the beach, Wiltshire encounters Case and accidentally reveals he knows about the luminous paint, alerting Case that his secret is exposed. At home, Wiltshire finds Maea, a powerful young chief with a personal rivalry with Case, who has decided to shift his business to Wiltshire. Wiltshire tells Maea the bush temple is a fraud and promises to destroy it that night; Maea agrees to inspect the ruins the next day and pledges secrecy.
That night, Wiltshire rigs the cellar with gunpowder and two dynamite bombs. Uma appears in the bush, having followed him to warn that Black Jack was spotted alerting Case. Wiltshire lights a slow-burning fuse. The blast is massive, scattering burning debris through the trees. Case, who has followed them, opens fire with a Winchester rifle. A shot breaks Wiltshire's leg; another wounds Uma in the shoulder. Wiltshire lies still, gripping his knife. When Case steps forward believing him dead, Wiltshire grabs him by the ankle, pulls him down, and stabs him repeatedly, declaring each thrust as vengeance for Underhill, Adams, and Uma. Case dies.
Wiltshire faints from blood loss. In the morning, Maea arrives with islanders and Mr. Tarleton, who sets Wiltshire's broken leg and takes formal depositions. Case is buried in the cellar hole. His will leaves everything, including Randall's assets, to a Samoan wife, who sells out to Wiltshire cheaply. Randall and Black Jack relocate and fail; Randall dies in a dynamite accident, and Black Jack is expelled for theft and reportedly killed by islanders in the western Pacific. Tarleton restores Wiltshire's standing, extracting a pledge that Wiltshire will deal fairly with the natives. Wiltshire prospers but feels uneasy about the pledge constraining his trade practices. In the closing, he reflects on his life with Uma, praising her as a generous and formidable wife. He has stayed in the islands for the sake of his mixed-race children, though he worries about his daughters' futures.