In his exploration of British imperialism, Robert Louis Stevenson paints a complex portrait of a society gripped by fear and operating without apparent rule of law. While he exposes the negative effects of individual greed and power-lust, he implies that such cases are aberrations and does not challenge the fundamental bases of European control. The novel’s main character, John Wiltshire, is far from perfect, but he shows positive character traits that increasingly distance him from the "bad" colonists, of whom Case is emblematic. Apparently intending his work as a realistic exposé of colonialism’s excesses, Stevenson elaborates on numerous unpleasant aspects, including deceit and hypocrisy. With the portrayal of Case, who distorts indigenous religion for his own benefit and has become thoroughly corrupt, the novel has generated comparisons with Joseph Conrad’s Heart of Darkness (especially in its character of Kurtz).

The moral and ethical progress of John Wiltshire is at the novel’s center. Wiltshire is initially selfishly concerned with his own business interests and quality of life. Believing that Case is a sympathetic compatriot—a fellow white Englishman far from home—and having negative views of religion, Wiltshire agrees to deceive an innocent young woman into believing they are legally married. His excuses are that such fake ceremonies are customary on the islands and that he does not place credence in the indigenous people’s Christian faith because he thinks the missionaries have tricked them into converting. It is a missionary, Tarleton, who awakens him to Case’s illegal and immoral activities. Once Wiltshire realizes he has been fooled, he determines to ruin Case. His change of heart is accompanied by the realization that a church-sanctioned marriage is important, so Tarleton marries him to Uma.

Stevenson also portrays the indigenous islanders as gullible and superstitious. Apparently unable to distinguish between the spirits derived from their religious beliefs and those that Case manufactures, they fall under the Englishman’s spell. Case’s power is compounded by an indigenous Christian pastor who supports Case’s version of events and thus extends the islanders’ fear, further enabling his control. The most powerful chief, Maea, opposes Case but must bide his time until an Englishman can support his efforts. Maea is guiltless in the physical destruction of Case’s secret lair and in his death, both of which are accomplished by Wiltshire, but as the author implies that these are heroic acts, he also puts them in the foreigner’s hands.