Astérix and Obélix have to win the Olympic Games in order to help their friend Lovesix (Stéphane Rousseau) marry Princess Irina (portrayed by Vanessa Hessler). Brutus (Benoît Poelvoorde) uses every trick in the book to have his own team win the game, and get rid of his father Julius Caesar (Alain Delon) in the process, but fails.
Infuriated by belittlements, Queen Cleopatra makes a deal with Julius Caesar: if the Egyptians build a palace larger than Caesar's palace in Rome in three months, he must acknowledge that Egypt was the greatest of nations. To perform this task, Cleopatra hires the architect Numerobis, on pain of death and much to the dismay of Pyradonis, Cleoptara's customary architect. He and his secretary Papyris discuss the druid Getafix (Panoramix in the original French), whose potion empowers its drinkers; and Numerobis goes in search of him.
Julius Caesar lands in Britain, where a small village still holds out bravely against the Roman legions. But the situation becomes critical and the villagers have not got long to live; that's when Jolitorax, one of the villagers seeks volunteers to go seek help from his second-cousin Asterix the Gaul and bring back a barrel of "magic potion" from the Breton village of the indomitable heroes.
Pour la première fois de sa vie, Panoramix manque de se tuer en tombant d'un arbre alors qu'il cueillait du gui. Se retrouvant avec un pied dans le plâtre, Panoramix réalise qu'il met le village en danger en gardant pour lui seul le secret de la potion magique. Il décide donc de partir à la recherche d'un successeur, à la surprise totale du village. Afin de ne pas se tromper dans le choix du prochain druide, il demande l'aide d'Astérix et Obélix pour l'accompagner, et contacte les autres druides de Gaule en envoyant un message par des sangliers.
A tribe of Vikings feel flustered because in every raid they find their target locations devoid of people. Cryptograf, an advisor to Viking chief Timandahaf, says that the enemies always disappear because "Fear gives them wings". Timandahaf excitedly concludes that they must find a "Champion of Fear", who can teach them to be great cowards so they can fly, making them invincible. The chief says he'll give anything to whoever can bring him this Champion of Fear. Cryptograf hatches a secret plan to seize power and tells the chief they'll find the Champion of Fear in Gaul.
Julius Caesar is celebrating his victory over all of Gaul, but Lucius Detritus has kept from him that one village has managed to resist them. Detritus travels to the garrison near the village where Caius Bonus (Crismus Bonus), the garrison's commanding Centurion, explains that the Gauls have a magic potion, which makes them invincible. Detritus decides to capture the potion for himself, and hearing that the clever Asterix and permanently invincible Obelix are the backbone of the Gaulish forces, attempts and fails to eliminate them.
After the spectacular defeat of a fresh centurion, Caesar sends the patrician Lucullus to capture Getafix and throw him off the edge of the world. In Gaul, Unhygienix lacks fresh fish (as always), which starts a fight, spilling Getafix's magic potion. Because fresh fish is a vital ingredient thereof, Asterix and Obelix go fishing while Getafix gathers herbs, accompanied by Dogmatix. In the forest, Getafix and Dogmatix are captured by Lucullus. Aboard a Roman galley bound for the "world's edge", they pass Asterix and Obelix, who pursue them, using Obelix as an outboard motor until he is diverted by the aroma of food from a passing pirate ship, and then blown off course by a violent storm. After a long pursuit, a dolphin brings Dogmatix (whom Lucullus had dropped overboard) and guides them to the Romans. Seeing the Gauls and mistaking the North American coast for the world's edge, the Romans launch Getafix from a catapult, then return to Rome, while Asterix and Obelix make landfall. In Gaul, Caesar lays siege to their village and waits for the last of the magic potion to run out.
After a heated argument with Julius Caesar over his lack of faith in the Egyptian people, Queen Cleopatra enters into a bet claiming that she can have a magnificent palace constructed for him in Alexandria within three months. She gives this enormous task to her best architect Edifis (despite his shortcomings), informing him that he will be covered with gold if he succeeds or thrown to the crocodiles if he fails. Daunted and distraught, believing the task to be insurmountable without some sort of magic, Edifis travels to Gaul to seek help from the famous druid Getafix.
After a group of legionaries is once again beaten up by the gauls, they imagine: "With such huge strength, they can't be human... they must be gods". Julius Caesar is informed, and laughs. He makes a decision with his council and goes to Armorica, to speak with Vitalstatistix. He gives the Gauls a series of 12 tasks, inspired by Hercules (but new ones, since the 12 Labours are outdated). Vitalstatistix assembles their best warriors, Asterix and Obelix, to do the job. The Roman Caius Tiddlus is sent along with them to guide them and check they complete each task.
After a brief introduction to the principal characters, the film follows Asterix as he encounters a group of Roman legionaries in the forest. Despite being significantly outnumbered, Asterix leaves the Romans beaten and bruised for daring to interrupt his wild boar hunt. Their state upon their return to camp prompts the leader Phonus Balonus to seek the secret behind the Gauls' seemingly superhuman strength. Phonus selects a volunteer (by means of a single round of musical chairs) to pose as a Gaul in order to infiltrate the village; the unlucky loser is a short, slack-tongued misfit named Caligula Minus. He is dressed in a wig, false moustache and traditional Gaulish dress and led in chains through the forest as a prisoner, awaiting rescue by the Gauls. Sure enough, Asterix and Obelix free Minus and believe his flimsy cover story that he is a Gaul from Belgium.
The movie begins when Obelix falls in love with the chief's niece, Panacea. His heart is quickly broken when he hears that she is due to be married to the handsome Tragicomix. The two lovers are walking in the woods when they are captured by a group of Romans led by a fresh new recruit looking to impress the Centurion of the Roman Camp. The Centurion is infuriated, knowing that the Gauls will attack. Panacea, Tragicomix and the new recruit are sent to the farthest outpost of the Roman empire (in the Sahara) while the Centurion and his troops prepare for the battle.