The story chronicles the efforts of King Leopold II of Belgium to make the Congo into a colonial empire. With a complex scheme of political intrigue, corruption and propaganda, he wins the assistance of one of the best-known explorers of the time, Henry Morton Stanley, as well as that of public opinion and of powerful states. Through the Berlin Conference and other diplomatic efforts, he finally obtains international recognition for his colony. He then establishes a system of forced labour that keeps the people of the Congo basin in a condition of slavery.
The book places King Leopold among the great tyrants of history. The death toll in the Congo under his regime is hard to pin down, both because accurate records were not kept and because many of the existing records were deliberately destroyed by Leopold shortly before the government of Belgium took the Congo out of his hands. Although Wm. Roger Louis and Jean Stengers characterize the earliest population and mortality estimates as "wild guesses", Hochschild cites many subsequent lines of inquiry that conclude that the early official estimates were essentially correct: roughly half the population of the Congo perished during the Free State period. Since the census taken by the Belgian government (after acquiring the Congo from Leopold) found some 10 million inhabitants, Hochschild concludes that roughly 10 million perished, though the precise number can never be known.
Hochschild profiles several people who helped make the world aware of the reality of the Congo Free State, including:
- George Washington Williams, an African American politician and historian, the first to report the atrocities in the Congo to the outside world.
- William Henry Sheppard, another African American, a Presbyterian missionary who furnished direct testimony of the atrocities.
- E. D. Morel, a British journalist and shipping agent checking the commercial documents of the Congo Free State, who realized that the vast quantities of rubber and ivory coming out of the Congo were matched only by rifles and chains going in. From this he inferred that the Congo was a slave state, and he devoted the rest of his life to correcting that.
- Sir Roger Casement, a British diplomat and Irish patriot, put the force of the British government behind the international protest against Leopold. Casement's involvement had the ironic effect of drawing attention away from British colonialism, Hochschild suggests. The Congo Reform Association was formed by Morel at Casement's instigation.
Hochschild devotes a chapter to Joseph Conrad, the famous Anglo-Polish writer, who captained a steamer on the Congo River in the first years of Belgian colonization. Hochschild observes that Conrad's novel Heart of Darkness, despite its unspecific setting, gives a realistic picture of the Congo Free State. Its main character, Kurtz, was inspired by real state functionaries in the Congo, notably Leon Rom. While Heart of Darkness is probably the most reprinted and studied short novel of the 20th century, its psychological and moral truths have largely overshadowed the literal truth behind the story. Hochschild finds four likely models for Kurtz: men who, like Kurtz, boasted of cutting off the heads of African rebels and sometimes displaying them.