The tale soon confirms that Calculus was indeed kidnapped by Chiquito, the Inca posing as a knife-thrower's assistant, but takes longer to explain how the seven explorers were inflicted. And the explanation produces an "I don't believe it!" from Tintin, a sign of the steps the series is taking. For the first time in the series there's an explicit resort to witchcraft and magic. Though the contents of the crystal balls themselves have a potentially scientific explanation, the use of representative wax figures to cast spells over the explorers and torture them cannot be explained this way. Nor, for that matter, can the clairvoyant or the convenience of the ball lightning in the previous part - or even the surprisingly accurate prophecy found in carvings by the explorers. Even more so than the rapid growth energy of the meteorite in The Shooting Star, this revelation takes the Adventures into a new realm of fantasy.
That's not to say the stories up to now have always been fully realistic, with Tintin and, especially, Snowy often performing incredible feats in an environment with a touch of exaggeration and parody about it. But there is a difference between amazing exploits in a basically realistic world and a resort to supernatural, especially as the extended nature of this saga means that it comes as a resolution to a long-standing mystery that has been hanging since the first part. And given that the explanation only comes on page 60 of 62, it feels very much out of the blue compared to what has come before. It's ultimately a lazy solution when a more chemical based one could have applied - e.g. the potion in the crystal balls induced both comas and paranoia, whilst the close proximity of the explorers in the hospital meant that that their reactions triggered each other to the point that their bodies' natural cycles synchronised, but they can be cured with another potion supplied by the Incas. Although still incredible, such an approach would have kept the story on the natural side of events. The clairvoyant, ball lightning and prophecy might be harder to handle though.
Away from the magic, the Incas are depicted as a sophisticated race, with a hidden civilisation in full flourish and an outreach into not only the rest of Peru but also the wider world, allowing them positions of power and influence to carry out their activities without being detected. They are a noble and proud people who have maintained their traditions and religion but it becomes clear all their hostile actions are to protect their society and traditions. Their isolation allows Tintin to fool them by exercising his right to choose the time of his execution and picking the moment of an eclipse which he then claims is a sign that he has the backing of the sun, but after this he is able to appeal to them with reason and get the explorers free from the spell.
Most of the rest of the tale is taken up with a chase and search through Peru, involving a sabotaged train journey, a trek through first the Andes mountains and then a jungle and finally a gruelling search through caves that contain a long-forgotten entrance to the Temple of the Sun. Throughout it Tintin and Haddock are aided by Zorrino, a young boy whom Tintin rescued from bullies, earning multiple gratitudes in the process. This also triggers one of the story's running gags as Haddock keeps irritating llamas who in turn spit in his face - but at the very end he gets his revenge by proxy on the species. The trek doesn't hold back from the dangers, both from a group of Incas and from natural hazards and predators. This leads to a succession of tense sequences, including an attack by a condor, avalanches, a giant snake and their pursuers. Especially memorable is the scene aboard a train where Tintin and Haddock find themselves alone in the rear carriage which suddenly breaks away and they have to jump out before it crashes over a canyon, a dramatic tense scene that has since found fame as the very start of the introduction to the 1990s cartoon.
Surprisingly little used in this story is the target of their quest, Professor Calculus. Initially seen drugged aboard the ship that arrives in Peru, he is then perpetually off panel until the very hour allotted for their execution. Throughout all this time - and the adventure seems to take place over a good month - he has been utterly oblivious to what is going on and seems to believe he has wound up with a part in a movie. Even less significant are the Thom(p)sons who come to a fork in the road with Haddock as they seek a temporarily missing Tintin and the detectives explore the wrong direction. They reappear towards the end of the album as they try a new method of searching for Tintin and Haddock, trying Calculus's method of dowsing. This results in a pendulum giving them some vaguely correct information about what is happening to their targets but they wind up in a comedic search across much of the world in comedic aside panels that help to mark the time as the story heads towards its climax.
Overall this is very much a quest and action tale that does a good job of showing another civilisation that could realistically have survived in the mountains and the search for them makes for a strong and tense adventure. However, the use of magic as the explanation for mysteries in the first part of the epic is a serious let-down for the series which has hitherto not gone in for such fantastical elements. It's because of this that the album is ultimately a disappointment after showing so much promise.