With Getafix away from the village at a druid convention, the Gauls take shelter from a fierce thunderstorm in the chief's house, fearing that the storm is a sign that Taranis, god of thunder, is angry with them. Their worries are ended when Prolix, who claims to be a traveling soothsayer, arrives and predicts that the storm will pass and that a fight will soon break out after reading the entrails of a fish - predictions which both come true. Most of the village is awed by these predictions, but Asterix remains skeptical, claiming that it was just a mixture of luck and realizing that fights break out regularly in the village (especially when Unhygienix's fish is discussed).
After the storm passes, the soothsayer takes up shop in the forest, making prophecies in exchange for goods. Curious as to why people keep taking their fish/chickens/etc with them for walks in the woods, Obelix (who had argued with Prolix while he was in the village) goes into the woods and finds the soothsayer. While Prolix manages to distract Obelix, Asterix is confused by everyone's behavior, and goes into the woods as well, only to find that the soothsayer has disappeared. In fact, he has been taken by a Roman patrol from the camp of Compendium, the centurion at which, Arteriosclerosus, tells Prolix they are under orders to arrest all Gaul soothsayers. Desperate not to be locked up, he tries to explain that he's not really a soothsayer, he's just a con man living off of Gaulish gullibility. This gives the commander an idea.
Prolix returns to the village and claims that they are now cursed, and their village will soon emit a foul stench that will turn all that breathe it a noxious hue. Fearing the curse, the Gauls (apart from Asterix and Obelix) flee the village to a nearby island. Prolix returns to Compendium and reports that the Gauls have left.
While the Romans take the village, Asterix and Obelix wander the forest and run into the returning Getafix. After telling him what's been going on, the druid gets an idea to discredit the soothsayer. Shortly after the Roman commander sends a messenger to report all of Gaul has been pacified, a foul stench erupts over the village, turning all who breathe it a noxious hue. Fearing pestilence, the Romans retreat. Cacofonix, who had snuck back into the village to retrieve his lyre, also smells it and returns to the villagers to report the prediction had come true.
Asterix, Obelix, and Getafix meet up with the other villagers and explain that the source of the stench was a potion that Getafix had brewed, not any divine punishment. The Gauls return home (sinking a pirate ship that had anchored for the night between the island and the village on the way); however, despite everything, a lot of the village wives still believe Prolix is a real soothsayer (because he told them things they wanted to hear). Asterix suggests that they prove otherwise by giving Prolix a surprise. So Getafix brews up an extra-large batch of potion and invites the women to join the men in raiding the Roman camps.
At the camp, Prolix is utterly perplexed by his prediction actually coming true, and perpetually badgered by Arteriosclerosus' belief that the aid of a powerful soothsayer could propel him to the throne. Both these troubles are repelled by an unexpected horde of Gauls trashing the camp. They return from their expedition to find some Roman officials (there to investigate the report of the village being taken) in the village. Vitalstatistix has them roughed up and tossed out.
As Arteriosclerosus prepares to have the soothsayer locked up, the officials that Vitalstatistix had beaten up arrive and punish the centurion for falsely reporting the capture of the village by demoting him to common legionnaire. Outraged by the failures of his soothsayer, Arterioscerolsus orders his adjutant to arrest him, only to be told 1: If Prolix is a fake, then the standing orders to arrest soothsayers don't apply, and 2: Since the centurion is no longer an officer, he has no business giving orders to actual officers. As the now former centurion is set to sweeping the barracks, Prolix is ordered out of the camp. He vows to give up soothsaying, but Taranis remains sceptical...
The Gauls return home triumphant and have a celebratory feast.
- Art Imitates Art: The scene of Prolix "reading" the fish is drawn to resemble the 1632 painting The Anatomy Lesson of Dr. Nicolaes Tulp by Dutch artist Rembrandt van Rijn, with the fish taking the place of the cadaver in the original painting.
- Authority Equals Asskicking: Or intelligence. The Optio says he doesn't get it, Arteriosclerosus responds that it's to be expected, "since I'm a centurion and you're an optio". The optio wanders away scratching his head, a legionary asks if he needs help, and gets told "no, you wouldn't understand, since I'm an optio and you're just an legionary". When Arteriosclerosus gets demoted, the Optio ignores his attempt at convincing him Prolix is a soothsayer because he is under him now.
- Berserk Button: Don't try to harm Dogmatix. Obelix will break your nose.
- Call-Back: When Prolix accidentally correctly predicts that a pair of dice will add up to VII, he explains that if he really were a soothsayer but wanted to prove himself a fake, he would have known the dice would add up to VII and would have predicted a roll of VIII instead. The none-too-bright Optio of Compendium is thoroughly confused by Prolix's defence, and in near the end of the book, he collars Prolix and orders him to make another prediction about a dice roll. This time, Prolix guesses VIII, but the dice add up to VII, which only confuses the Optio further.
- Delusions of Eloquence: Arteriosclerosus' Optio tries to sound important and intelligent when he talks, but his tendency to drop his Hs, flout subject-verb agreement ("Orders is orders"), and expand his sentences with liberal visits to the Department of Redundancy Department make it clear how dim he really is. His report to Arteriosclerosus after first arresting Prolix illustrates this perfectly:Optio: On proceeding on patrol, for which you gave the orders to proceed with, we found this 'ere individual in a clearing, and after a caution he made a statement what we were not very satisfied with.
- Deus Exit Machina: Getafix isn't present for the first half of the book because if he was, the plot couldn't unfold the way it does (with Asterix as the Only Sane Man).
- Dramatic Irony: On the page discussing soothsayers and their unreliability, an augurer is telling Julius Caesar that as long as Brutus is near him, he has nothing to fear (Brutus is even playing with a knife). Of course, we all know how accurate that was... Alternatively, the prophecy may be exactly correct. As long Brutus is on Caesar's side, he won't fall, but the moment he changes side... Then fall, Caesar.
- Embarrassing Nickname: In the early days of their courtship/marriage, Impedimenta used to call Vitalstatistix "Piggywiggy". When Prolix tells her that Vitalstatistix will move to Lutetia to go into business with her wealthy brother Homeopathix (seen in the previous volume), she revives the nickname, much to Asterix and Obelix's amusement.
- Gone Horribly Right: Every time Prolix needs to prove that he can/can't see the future, it goes exactly the way he doesn't want it to.
- Historical Injoke: Once more involving Brutus. A short parenthesis about Soothsayers and how they are nothing but charlatans depicts one telling Julius Caesar that he will have nothing to fear as long as Brutus is near him.
- Karma Houdini: In the end, Prolix gets away, without a scratch. However he does seem to have been a bit humbled by the experience and it's ambiguous on whether he will go back to his scammer ways. In the later album The Asterix and Obelix Birthday Book, we find Prolix up to his old tricks making preposterous predictions during Asterix's and Obelix's birthday party, to which he has inexplicably been invited (along with Squareonthehypotenus from The Mansions of the Gods, Redbeard and his crew, and Julius Caesar himself!); and while the Gauls are mostly tolerant, they are clearly skeptical.
- Morton's Fork: Prolix manages to impale himself on Morton's fork within seconds of being hauled up before Centurion Arteriosclerosus. He tries his usual method of playing on his victims' vanity by claiming to foresee the centurion's impending promotion, only to be told the Romans are under orders to arrest all Gaulish soothsayers. He tries to backpedal, claiming to be a charlatan... in which case he will be punished for insulting the centurion by pretending to foresee his promotion. Fortunately, he is able to fast talk his way out of this predicament... for a while.
- No Name Given: Despite being a prominent character in the story starting with Prolix's arrest, Artierosclerosus's rather dim-witted Optio is never addressed by name.
- No-Sell: Unhygienix is the only one unaffected by Getafix's foul-smelling mixture of gasses, because, considering his profession, he is used to such bad odors.
- Pet the Dog: After spending all the comic trying to lock Prolix, the Optio simply tells him to get lost.
- Phony Psychic: Prolix. The narration states that most so-called "prophets" were just fast-talkers taking advantage of people's gullibility. Which is then immediately parodied, as one of them is shown forecasting a "reasonable" view of the future (a picture of Uderzo's villa), while another is shown "saying complete rubbish" (a photograph of the then-under construction La Défense skyscraper district in Paris).
- Pun: Despite claiming to despise easy puns, Getafix calls the cauldron of stinking potion "the pollution to all our problems".
- Running Gag:
- The traditional fight over the quality of Unhygienix's fish breaks out three times over the course of the book. Asterix even lampshades it by pointing out that fights break out every time people discuss the fish. Unhygienix denies this, and Fulliautomatix claims that it wouldn't happen if the fish was fresh. Unhygienix hits Fulliautomatix with a fish, and another fight breaks out.
- Every time one of Prolix's predictions appears to come true following his arrest by the Romans, the Optio asks, "Do we lock him up, then?"
- The pirate ship gets sunk again. And just like in Asterix in Switzerland, the unlucky pirates prove that are perfectly capable of doing so without Asterix and Obelix around (the parenthesis shows a Soothsayer predicting them that it's safe to set sail, but they end up in a severe storm that sinks their ship).
- Sherlock Scan: Subverted, since most of what the seer predicts is either blatantly obvious or what he believes his audience wants to hear. When the Optio is reminded of where he used to live due to the hideous odor now floating over the village, Prolix innocently asks if he used to live near a tannery (as Prolix did), which causes the optio to try to arrest him for being a real seer.
- Springtime for Hitler: Prolix tries to prove that he is a fake by making a prediction on the outcome of a dice roll. He predicts a combined roll of VII, and believes that as he always loses at gambling, he should be safe in making an inaccurate prediction. However, the dice land on V and II, which add up to VII, so Arteriosclerosus orders his arrest. Of course, it was not a good choice, because seven is the most common result of rolling two six-sided dice (and not knowing that may explain why he always loses at gambling). Even worse in the animated adaptation where he predicts the coin toss would be neither heads nor tails, and the coin gets stuck in an amphora on the edge side.
- Tempting Fate: After being thrown out of the Roman camp, Prolix declares an end to his days as a soothsayer, and says that if he is lying, may Taranis strike him with a thunderbolt. Cue the sudden storm...