Claiming that the indomitable Gaulish village is proof that Caesar is incapable of maintaining order within his Empire, the Roman Senate refuses to provide him with the funding he needs to launch further wars of conquest until he consolidates Rome's power in the territories he's already taken. As Caesar meets with his councilors to figure out a way to subdue the village, one of them suggests that their true strength comes not from the magic potion but from their unity. Another one suggests the perfect agent to turn the Gauls against each other - Torturous Convolvulus, a man so inherently disruptive that his neighbors managed to get him thrown to the lions - at which point the lions ate each other. Caesar sends for the man and is quickly impressed by the way he gets the guards escorting him and the councilors at each others throats - and nearly gets Caesar to join in after speaking a mere two sentences. Promising Convolvulus a pardon and a rich reward if he succeeds in his mission, Caesar puts him on a ship to Gaul, which nearly sinks itself on the voyage because of the chaos he causes by his mere presence (And getting the pirates to sink themselves after a short conversation, proving that they don't need the Gauls to look like fools, making a good job of it themselves).
Meanwhile at the village, everyone is preparing for the Vitalstatistix's birthday celebration. As the chief practices his birthday speech, Impedimenta points out that claiming to be surprised by what the villagers give him is ridiculous, and they give him the same junk (a mixture of swords, shields, menhirs and stuffed fish) every year. Then Convolvulus arrives carrying a valuable vase (taken from the tent of the legion commander of the base he's operating out of), which he says is a gift from Rome to the most important Gaul in the village. The whole village gathers to see Rome offer tribute to a Gaul, only for Convolvulus to walk past Vitalstatistix and give the vase to Asterix. After the Roman leaves, the village wonders why the vase was given to Asterix and not the chief. A gathering of the village wives (including the wives of Geriatrix and Fulliautomatix, who are introduced in this volume) combines the facts that Asterix is a close friend of Getafix and the Roman's gift and concludes that the druid told Asterix the secret of the magic potion and that Asterix sold it to the Romans.
The rumor of Asterix giving the potion to the Romans reaches the garrisons surrounding the village, and Convolvulus starts the next stage of his plan. He has Magnumopus, an enormous legionnaire with a tiny head, ambush Geriatrix and leave his helmet behind. The villagers take Geriatrix being beaten up by a tiny soldier (according to the evidence of the helmet) as further evidence that the Romans have the potion. Vitalstastix sends Fulliautomatix and Unhygenix to get confirmation, and they see all the Romans in the camp lining up in front of a cauldron. When they see a tiny soldier drink the 'potion' (actually water) and punch out the much larger Magnumopus (who is only pretending to be knocked out), they run back to the village with the news.
With eyewitness evidence that the Romans have the potion, the village confronts Asterix and Getafix. Asterix is outraged at the accusation that he sold the secret of the potion to the Romans. Getafix is outraged by the allegation that he told it to Asterix in the first place, as the potion is a secret that can only be passed orally from druid to druid. The two of them leave the village, accompanied by Obelix, the only person in the village who still has faith in Asterix (other than Cacofonix, who spends the entire book oblivious to what's going on).
The three of them confront Convolvulus and say that they despise him and that they're going to leave the area. As Convolvulus runs to the legion commander and explains that the rest of the village is defenseless without them, the Gauls steal the Roman's 'potion' and fight their way back to the village. As the Romans gather their full force to attack the village, Getafix demonstrates that the Romans didn't really have the magic potion and makes a batch of the real stuff. The potion is completed and distributed at the same time as the Romans finish assembling, and the battle is joined.
The battle ends with the four Roman armies (as well as the pirates, who were only passing by and were attacked simply because they were there) being defeated. With the legions crushed, Asterix confronts Convolvulus... and thanks him for setting up the legions, giving back the vase as payment for selling out the Romans. The Roman commander refuses to listen to Convolvulus' protests that the Gauls were lying and orders him put in chains and sent back to Rome to face treason charges--assuming Convolvulus' continued discord-sowing doesn't get himself and the crew drowned.
With the Romans defeated and the village reunited, Asterix suggests that they hold a second birthday party for the chief, to make up for the one that turned out so disastrously earlier in the book due to Convolvulus' scheming. However, Getafix feels the village should be taught a lesson, and conspires with Asterix and Obelix; later on, villagers see Obelix carrying Asterix on a shield, and rumors lead to the assumption that Asterix has somehow become the chief. However, it is revealed that the shield is a birthday gift, which Asterix was 'testing' for Vitalstatistix. With that settled (and the others feeling a bit foolish), the village has its feast and Vitalstatistix gives his speech.
* AppleOfDiscord: Convolvulus' specialty. The vase given to Asterix is a specific example.
* AteItAll: Asterix and Obelix are sitting down to a dinner of wild boar when Convolvulus comes to his door; as the Roman leaves, he loudly thanks Asterix for the boar. Asterix gets up to deal with him, and he's not out for one minute before returning to find that Obelix has devoured both their boars, ruining his attempt to explain himself to a suspicious Vitalstatistix.
* BeeAfraid: Three legionaires hear Gauls coming and hide in a tree, which happens to have a hornet's nest. They come back to the camp covered in welts.
* BelievingTheirOwnLies: More like 'believing the civil servant's lies'. The Legionaries are new to the psychological battle, and several times believe they have the Magic Potion.
* ComicallyMissingThePoint: Asterix and Obelix are sent to hold off the Roman's advance units while Getafix brews the magic potion. After the usual CurbStompBattle, Obelix says that they need to press the Romans to stay - they can't hold them off if they're running away!
* DumbMuscle: Magnumopus, which Convolvulus uses to his advantage. The Romans of Aquarium try to adopt this as a whole, but Platypus puts a stop to it.
* EveryoneHasStandards: Fulliautomatix ends up slugging Cacofonix in the face after Getafix whacks him on the foot, because he can't bring himself to hit someone in their nineties.
* FacePalm: What inevitably happens when a ManipulativeBastard meets [[ArmedFarces military intelligence.]]
* GenreSavvy: Unhygenix is well aware of when the fish fights are going to start, and as soon as trouble brews he brings his fish in.
** Convolvulus stages an elaborate show to convince the Gauls that they have the magic potion. Unfortunately, the Romans believe it as well...
** Later, this is how the Gauls beat Convolvulus. By the end of the book, he has established an atmosphere in which everyone associates him with tricks, ploys and double-crosses. After the Romans have been pounded into a fine paste, Asterix and Obelix simply walk up to Convulvulus in the presence of the centurion and thank him fervently for all his help, giving him the vase as "payment". Convulvulus tries to convince the centurion that it's a trick, but it's too late; since he's already shown that he's a ManipulativeBastard with zero conscience, it's all too easy for the centurion to believe that he would sell the Romans out.
* GossipEvolution: Impedimenta and her circle tend to make tense situations worse with their speculations.
* HatePlague: Convolvulus' mere presence is enough to make people hate each other (and it transcends species; when he was sentenced to execution in the circus, the lions ended up eating each other instead) and he simply needs a few words to separate great friends.
* HoistByHisOwnPetard: The vase Convolvulus gave to Asterix is used to frame him.
* IgnoredEnemy: The Pirates prepare to attack the Roman Galley carrying Convolvulus to Gaul...but the Romans are too busy arguing to pay them any mind. Then Convolvulus works his evil on the Pirates (by claiming that he personally bought off Baba the lookout), who sink themselves as a result of in-fighting.
* JerkassHasAPoint: Fulliautomatix is being a bit of a dick when lecturing Unhygenix about the smell of fish as they spy on the Romans, but it is exactly that that tips the Romans off that they're being watched.
* JustShootHim: Discussed during Caesar's meeting, throwing the full weight of the Roman army could take care of the Gauls, magic potion or not, but it would weaken the Roman Empire's borders.
* OnlySaneMan: Aside from Getafix (the sole villager to never fall influence to one of Convolvulus' speeches), Asterix, and Obelix, Unhygenix, oddly enough, serves as this for a good portion of the story. He buys into the belief that the Romans have the potion due to observing their trick, but for the rest of the story he remains on Asterix's side and tries to break up the fish fights.
** Characters under Convolvulus' influence have green SpeechBubbles. When Obelix' anger subsides, his bubbles become progressively whiter.
** When Vitalstatistix finally gives his birthday speech, the speech bubble for the warm, emotional quote is coloured pink.
* ThePowerOfFriendship: Not even Convolvulus can break apart Asterix and Obelix's friendship, and they make up almost immediately.
* ProperlyParanoid: Near the beginning, Getafix worries that they've had so much good fortune - plenty of boars, no trouble from the Romans, Cacofonix losing his voice - that trouble must be brewing. Cut to the ship carrying Convolvulus.
** Magnumopus believing that 'Psychological Warfare' means hitting someone over the head with a club.
** Vitalstatistix falls off his shield even more often than usual, and goes from being angry about it to simply being mentally exhausted.
* ShoutOut: Centurion Platypus is clearly French actor Lino Ventura. Though a modern reader would be forgiven for confusing him for a caricature of Sylvester Stallone.
* SurroundedByIdiots: Convolvulus has problems explaining to the legionaries that they don't have magic potion.
* VitriolicBestBuds: The entire village, as lampshaded by Caesar at the start of the book; for as mad and prone to arguing and fighting as they are, they retain an unbreakable bond. This is proven true by the end of the story, as while Convolvulus's schemes did strain and jeopardize that bond, not even he could break it in the end.
* WhoWouldWantToWatchUs: After the vase is given to Asterix, Impedimenta tells her husband that if someone ever wrote about their village, they wouldn't call it ''The Adventures of Vitalstatistix the Gaul''.
* WinsByDoingAbsolutelyNothing: Convolvulus is presented to Caesar as being able to spread discord and infighting without even trying. He is brought in (by two arguing guards), and as soon as he arrives the counselors descend into ratting each other out, mudslinging, and namecalling, with Convolvulus pointedly looking smug as he hasn't uttered a single word and already caused a brawl.