This book is controversial and it would be wrong not to address the reasons behind that. But it's also necessary to consider the other points with this book as well and so let's get them out of the way first. This is the earliest of the Adventures to come in the standard 62-page colour format but there's still some of the oddness of the early series, with familiar supporting characters still yet to appear (bar a one panel cameo by Thompson and Thomson who have been added in in the redrawing) and at times Tintin is able to understand Snowy, rather than the dog merely commenting on the events. Tintin voyages to what was then the Belgian Congo (which later gained independence and became the Republic of Congo, not to be confused with the current Republic of Congo which was the French Congo, then the ex-Belgian one became the Democratic Republic of Congo then it became Zaire then it became the Democratic Republic of Congo again - you can see why the English translation at least predominantly just calls the place "Africa", title aside) to hunt big game and meets many locals on the way. A stowaway on the ship called Tom follows and tries to kill him, stirring up tensions in local tribes, but there really isn't much more to it than that. Tom's motivation is ultimately revealed to be that he's working for Al Capone, who is trying to take over the African diamond market and worried about that Tintin will discover it. Either it's the kind of absurd elevation of the hero into a skilled operative feared by criminals the world over that just doesn't ring true at this stage or Capone is targeting every single reporter who goes to one of the biggest places in the whole world. And that's it - there's no real plot to the story. It's just an extended travelogue. Even the other characters just drift in and out, most notably Coco, the young Congolese who serves as Tintin's "boy" servant in the early part of the story, rescuing him from a village before disappearing altogether.
Tintin in the Congo is an example of how the early stories were originally written for weekly publication, with subsequent collections (and redrawings) a bonus rather than the targeted format. As a result, the story is really a series of incidents with no real development or mystery beyond Tom's instructions and even then there's little at first to suggest he's anything other than a criminal on the run. As this was originally published a few pages at a time over the course of a year (the modern version also packs more panels into individual pages), it's understandable that things were kept simple for readers, but this really isn't some great classic Tintin adventure to get excited about. And then there's the precise setting.
The Adventures of Tintin were produced over a period of more than fifty years and it's invariable that they often reflect the attitudes of the time and sometimes those attitudes date a great deal. Now I'm acutely conscious that nearly all the criticism of this book on the web is from western countries. I've seen endless claims and counter-claims about how the book is regarded in the Democratic Republic of Congo itself or for that matter in wider Francophone Africa. Equally I'm aware that big-game hunting has its defenders not just in the west but also in Africa. It would be completely wrong to imply that all the controversy is really about how at ease western commentators feel about this book - there have been complaints by many Congolese from ministers to the student who filed a case in Belgium - but one has to be careful about imposing the values of western critics as though they speak for all.
To cut to the chase, the portrayal of the Congolese is incredibly patronising, treating them as primitive children. Notably this extends to power structures both amongst the ship's crew at the start of the story and the police at the end. Blacks may work in both, but the officers are uniformly white, a casual example of colonial hierarchies. In the Congo itself the Congolese are either spear carrying tribesmen & women or dressed in caricatures of "Sunday best" clothing as though they're trying to emulate the Belgians. Tintin and Snowy are treated as heroes upon their arrival and the final page shows a village where they have been elevated to deities. The most sophisticated help that Tintin receives comes from fellow white men - missionaries or explorers with sophisticated technology. The Congolese are instead childlike, speaking in broken English (even to each other!), easily impressed by simple technology, too lazy to do much without prompting, prone to manipulation and rapidly changing sides and more. In one scene Tintin takes a school class and none of the pupils can answer a basic maths question. This is classic imperialist justification, showing a "sophisticated" white man bringing "civilisation" to "primitives". The discomfort is not helped by Hergé's visual depiction of black characters with wide eyes (especially compared to white characters) and giant lips. Whatever the intent, it's impossible to deny that this is a stereotypical portrayal that would be unacceptable in modern literature.
The story also sees Tintin going on big game hunting, shooting down all manner of animals simply for their trophy value if even that. At one point, he even kills a monkey just to take its skin as a disguise to fool other monkeys! A sillier scene in the original French, both black and white and colour, editions involved drilling a hole in a rhinoceros and dropping in a stick of dynamite but for the colour Scandinavian editions this was replaced with a page where the rhino knocks the gun and is frightened by the noise of the resulting shot; this is the page used in the English-language colour edition. The big game hunter was a familiar figure in western culture in the early part of the twentieth century but in more recent years there's been such a backlash and various legal and commercial restrictions on either the hunting itself or the transportation of the resulting trophies, to say nothing of the online attacks every time someone posts a picture posing with a dead animal. At times the hunting incidents show a rather silly form of humour, such as when Tintin fires fifteen shots at what he thinks is a single antelope he keeps missing and discovers he's killed fifteen of them or the aforementioned encounters with monkeys and the rhinoceros. He also tries taking some film of animals including a scene where he dons a giraffe suit in order to get close enough to film two real giraffes. This is a book that's not sure if it's trying to be a Loony Tunes cartoon or a serious depiction and the result just feels weird.
How far one can accept the awkward parts of this book as being of its time is a subjective matter. But it is a very outdated portrayal of the Congolese that doesn't sit easily and it's hard to disagree with English-language publishers' decisions to generally skip Tintin in the Congo or else only make it available as a collectors' item. The story itself is incredibly thin and completely missable, as are many of the sillier moments. This is for completists only.