Tintin goes to visit Captain Haddock and Professor Calculus at their new home in Marlinspike Hall. On the train, he reads about a team of seven explorers led by Professor Sanders-Hardiman (Sanders-Hardmuth in the original French) returning home after a two year expedition in the Andes where they discovered the tomb of the ancient Incan ruler Rascar Capac, with the latter's mummy in tow. A strange man compares them to the Egyptologists who unearthed Tutankhamen's tomb and claims the same fate will befall them. Later on, Tintin attends a music hall with Haddock and is surprised to see General Alcazar, who since The Broken Ear was deposed and now is performing as a knife thrower. Another act by a psychic also mentions the expedition and warns its members are in grave danger.
Concurrent with these events, each member of the Sanders-Hardiman expedition starts falling into a mysterious coma, one by one. Thompson and Thomson are investigating and reveal that the only clue present at each scene are fragments of crystal spheres, which apparently contained some sort of mysterious gas that caused the comas. Despite the efforts of Tintin, Haddock, and the Thompsons, they are unable to stop the attacks and finally decide to personally guard the last explorer, Professor Tarragon (Bergamotte in French), an old friend of Professor Calculus, who accompanies them to the former's house. After Rascar Capac's mummy disappears from the house in a mysterious bolt of ball lightning, Tarragon claims that this is the culmination of a prophecy: Rascar Capac has "returned to his element" and will go after Tarragon personally. Despite the escort, Tarragon succumbs to the coma as well, becoming the last of the seven. To make matters worse, Professor Calculus is kidnapped after finding the mummy's bracelet.
Tintin and Haddock chase the kidnapper and manage to track him to the Pachacamac, a ship bound for Peru. Tintin also discovers that Alcazar's assistant Chiquito, one of the last descendants of the Incas is also involved. Tintin and Haddock depart for Peru to rescue Calculus, closing part 1 of the two-part story that concludes in Prisoners of the Sun.
The Seven Crystal Balls is the last story that Hergé produced as a daily comic strip for the "Stolen Soir" during the German occupation of Belgium. It was at first left unfinished after the liberation of Brussels restored Le Soir to its rightful owners and staff and Hergé was briefly jailed as a collaborator. Starting in 1946 was serialized once more and completed (with an extensive rewrite of the ending as far as it had been done in 1944) in the new comic magazine Tintin and then produced in album form. This enabled Hergé to take out old panels, add new ones, polish up the art and text and to change the layout without being bound by the constraints of the format used in the newspaper. This is most noticeable in the classic backstage slapstick sequence with Haddock in the music hall. A reprint of the 1943-1944 black-and-white strips with extensive comments and background information by Philippe Goddin was produced by Editions Moulinsart in 2012 under the title Les mystères des 7 Boules de cristal.
- And I Must Scream: The seven explorers themselves, especially Professor Tarragon.
- Ash Face: First Snowy and later Tintin get ashes into their faces when snooping around the chimney.
- Call-Back: See below.
- Continuity Lockout: An extremely minor example, but when our heroes are contacting the explorers to warn them of the curse, Professor Cantonneau greets Tintin on the telephone as an old acquaintance ("My dear Tintin"). This is because Cantonneau was also one of the scientists aboard the Aurora in The Shooting Star. It's a nice little grace note, but perhaps too subtle, as there isn't a word of explanation as to how the professor already knows Tintin. However, when the story first appeared it was only about three years since The Shooting Star was serialized and less than two since it was republished as a colour album, and besides the sequence in Crystal Balls is so short (six panels) that it does not merit the extra space needed to explain that Cantonneau has met Tintin and Haddock before.
- Curse: The explorers and archaeologists all fall victim to a curse due to desecrating a tomb. Subverted in that it requires human agents to make the curse come true for them.
- Dangerous Windows: Two of the members of the Sanders-Hardiman expedition fall victim to crystal balls hurled through their windows. Professor Cantonneau is attacked this way just as Tintin tries to warn him not to go near the window. Dr. Midge is then placed under police guard, but the police in question are Thompson and Thomson, and they try to guard everything but the window. With predictable results. To be precise, the two of them freak out over a brown paper package, fearing it contains a bomb or pathogen, and both go into another room to examine it, leaving the professor completely unguarded. The package has a butterfly specimen in it.
- Fainting Seer: Mrs. Yamilah, a stage fortune teller, tries to look into the fate of a woman married to one of the explorers. She gets a glimpse into the fate of the husband and seems physically affected.
- Fireballs: A huge fireball of lightning crashes into the protagonists' household. What some readers may be amazed to know is that this phenomenon actually has some basis in reality. Ball lightning is one of the least understood meteorological phenomena known to man. Essentially a ball of light that occurs during a thunder and or lightning storm, these little (or huge) buggers have been known about since antiquity. Nobody knows exactly what the stuff is though, other than a ball of light with electric properties. Its reported characteristics vary wildly - sometimes it can pass through people without harming them, sometimes they discharge massive amounts of electricity, even up to a normal lightning strike.
- Gratuitous Spanish: General Alcazar greets Tintin with a mix of English and Spanish lines.
- High-Class Glass: In the previous adventure, Red Rackham's Treasure, Captain Haddock had acquired a treasure through inheritance. His newfound wealth is indicated in this album by his residence in a mansion and by his new habit of wearing a monocle. Not being accustomed to them, he loses a lot of them.
- Impoverished Patrician: Subverted. Rupac Inca Huaco takes on the identity of Chiquito, a seemingly impoverished descendant of the old Inca rulers and now working as a traveling performer, to infiltrate Belgium and strike at the archaeologists without being noticed. Prisoners Of The Sun reveals that he's actually a high-ranking priest among the Incas, and they still have a lot of wealth and power.
- Knife-Throwing Act: General Alcazar, exiled in this book, has his own stage act. After creating a full Knife Outline around his Indian partner, he asks an audience member to blindfold him. His blind throw hits right in the center of the tiny disc-shaped target held over Chiquito's chest.
- A Mythology Is True: In-story example. The curse of Rascar Capac is scoffed at by the rational characters. Until it turns true to the letter.
- Nightmare Sequence: Tintin and his companions all have the same nightmare: that they are visited by the Inca mummy Rascar Capac who enters their bedroom by night and then throws a crystal ball on the floor.
- No Communities Were Harmed: The ending of the story is partly set in the French ports of Saint-Nazaire and La Rochelle, which Tintin and Haddock reach from Belgium by car. As the English translation relocates the home setting to England (Marlinspike in Marlinshire), it renames the ports Westermouth and Bridgeport, but the animated adaptation uses the original locations.
- No More for Me: Haddock pours himself a drink of rum as he swears before a portrait of Professor Calculus to find his missing friend. When the portrait starts speaking to him, Haddock decides he's had enough and pours the drink out a window.
- Real Fake Door: Haddock runs into a wall behind what turns out to be a prop door at the theater.
- Real Life Writes the Plot: Professor Tarragon's house really exists in Brussels. During the Nazi occupation of Belgium, when this story was made, Hergé and his assistant Bob De Moor went to the house to draw sketches of it. They had just finished and were about to leave when they saw a car full of SS soldiers drive to it in the distance. Apparently, it was used as one of their headquarters! When Hergé told this anecdote to a journalist after the war he added: "If they had turned up a little earlier they would've probably have spotted our suspicious activity and forced us to explain what we were doing there."
- Really Royalty Reveal: When Tintin meets Alcazar at the port, he asks him if his now missing partner was a real Indian. Alcazar replies that Chiquito was not only a full-blooded Quechua, but an actual descendant of the Inca rulers called Rupac Inca Huaco.
- Ripped from the Headlines: Much like Tintin: Cigars of the Pharaoh the plot was inspired by the so-called curse of the mummy surrounding the opening of Pharaoh Tutankhamun's tomb in 1922, only with the action brought to Peruvian mummies instead of Egyptian ones.
- Roadside Wave: Haddock gets splashed wet by another car in the road. Cue his Angry Fist-Shake.
- Rule of Seven: Seven cursed explorers, seven crystal balls hurled at them and placing them in comas.
- Stealth Pun: Captain Haddock walks into a bar...ouch.