Amid various encounters with the native Congolese people and wild animals, Tintin unearths a criminal diamond smuggling operation run by the American gangster Al Capone.

In 1946, Hergé re-drew and coloured Tintin in the Congo in his distinctive ligne-claire style for republication by Casterman, with further alterations made at the request of his Scandinavian publisher for a 1975 edition.

Tintin captures a criminal who tried to rendezvous with the stowaway and learns that "A.C." is none other than the American gangster Al Capone, who is trying to gain control of African diamond production.

He based the story largely on literature written by missionaries, with the only added element being that of the diamond smugglers, possibly adopted from the "Jungle Jim-type serials".

On the afternoon of 9 July 1931, Wallez repeated the publicity stunt he had used when Soviets ended by having a young actor, Henry de Doncker, dress up as Tintin in colonial gear and appear in Brussels and then Liège, accompanied by 10 African bearers and an assortment of exotic animals hired from a zoo.

[28] For example, in the scene where Tintin teaches Congolese school children about geography, he states in the 1930–31 version: "My dear friends, today I'm going to talk to you about your country: Belgium!"

[28] Hergé also changed the character of Jimmy MacDuff, the owner of the leopard that attacks Tintin, from a black manager of the Great American Circus into a white "supplier of the biggest zoos in Europe".

[28] In the 1946 colour version, Hergé added a cameo appearance from Thomson and Thompson, the two detectives that he had introduced in the fourth Tintin story, Cigars of the Pharaoh (1932–34), which was chronologically set after the Congolese adventure.

[17] Biographer Benoît Peeters opined that Tintin in the Congo was "nothing spectacular", with some "incredibly cumbersome" monologues, but he thought the illustrations "a bit more polished" than those in Land of the Soviets.

[33] Believing the plot to be "extremely simple", he thought that Tintin's character was like a child manipulating a world populated by toy animals and lead figurines.

[26] Michael Farr felt that, unlike the previous Tintin adventure, some sense of a plot emerges at the end of the story with the introduction of the American diamond-smuggling racket.

[21] Philippe Goddin thought the work to be "more exciting" than Land of the Soviets and argued that Hergé's depiction of the native Congolese was not mocking but a parody of past European militaries.

[35] Simon Kuper of the Financial Times criticised both Land of the Soviets and Tintin in the Congo as the "worst" of the Adventures, opining that they were "poorly drawn" and "largely plot-free".

[36] Farr saw the 1946 colour version as poorer than the black and white original; he said it had lost its "vibrancy" and "atmosphere", and that the new depiction of the Congolese landscape was unconvincing and more like a European zoo than the "parched, dusty expanses of reality".

[11] Peeters took a more positive attitude towards the 1946 version, commenting that it contained "aesthetic improvements" and "clarity of composition" because of Hergé's personal development in draughtsmanship, as well as an enhancement in the dialogue, which had become "more lively and fluid".

[37] In his psychoanalytical study of the series, Jean-Marie Apostolidès highlighted that in the Congolese adventure, Tintin represented progress and the Belgian state was depicted as a model for the natives to imitate.

[39] Literary critic Tom McCarthy concurred that Tintin represented the Belgian state, but also suggested that he acted as a Christian missionary, even being "a kind of god" akin to the character of Kurtz in Joseph Conrad's Heart of Darkness (1899).

[46] Laurence Grove—President of the International Bande Dessinée Society and an academic at the University of Glasgow—concurred, remarking that Hergé adhered to prevailing societal trends in his work, and that "[w]hen it was fashionable to be a colonial racist, that's what he was".

[45] Comic book historian Mark McKinney noted that other Franco-Belgian comic artists of the same period had chosen to depict the native Africans in a more favourable light, citing the examples of Jijé's 1939 work Blondin et Cirage (Blondy and Shoe-Black), in which the protagonists are adopted brothers, one white, the other black, and Tif et Tondu, which was serialised in Spirou from 1939 to 1940 and in which the Congolese aid the Belgians against their American antagonists.

[52] Alain Berenboom, lawyer for both Moulinsart, the company which controls Hergé's estate, and Casterman, the book's publisher, argued that the cartoonist's depiction of the Congolese "wasn't racism but kind paternalism".

Berenboom said that banning it would set a dangerous precedent for the availability of literature by other historical authors, such as Charles Dickens or Jules Verne, which also contain stereotypes of non-white ethnicities.

[52] In February 2012 the court ruled that the book would not be banned, deciding that it was "clear that neither the story, nor the fact that it has been put on sale, has a goal to ... create an intimidating, hostile, degrading, or humiliating environment", and that it therefore did not break Belgian law.

[53] In July 2007, British human rights lawyer David Enright complained to the United Kingdom's Commission for Racial Equality (CRE) that the book was sold in the children's section of Borders bookshop.

The CRE called on bookshops to remove the comic; responding that it was committed to letting its "customers make the choice", Borders moved the book to an area reserved for adult graphic novels.

[56] The media controversy increased interest in the book, and Borders reported that its sales of Tintin in the Congo had been boosted 4,000%, while it also rose to eighth on the Amazon.com bestseller list.

[57] Publisher Egmont UK also responded to racism concerns by placing a protective band around the book with a warning about its content and writing an introduction describing its historical context.

[54] Tintin in the Congo also came under criticism in the United States; in October 2007, in response to a complaint by a patron, the Brooklyn Public Library in New York City placed the graphic novel in a locked back room, only permitting access by appointment.

[58] Tintin in the Congo became part of a drawn-out media debate in Sweden after national newspaper Dagens Nyheter reported on the book's removal from a children's library in Kulturhuset in Stockholm in September 2011.