Frustrated with the Gauls' ongoing rebellion, Julius Caesar hatches a plan to subdue them: build a lavish city of apartments called The Mansions Of The Gods in the forest near their village, a move intended to absorb the Gauls into Roman civilization.
During a boar hunt, Asterix and Obelix discover the construction site. The project is led by Squaronthehypotenus and a Roman legion commanded by Centurion Somniferus who orders the slaves to clear the land. The Gauls counter by using magic acorns from Getafix the druid, which sprout into full trees when planted.
Upon learning that Squaronthehypotenus threatens to work slaves to death, [Asterix] and [Obelix] invade the Roman camp to free the slaves and provide them with Getafix's magic potion as a means to escape. The plan backfires: the slaves use the potions to speed construction, bid for the same pay as legionaries, and secure apartments for themselves, while the legionaries go on strike as well.
The Gauls plan to attack but discover the mansions are already populated by Roman civilians. Realizing they cannot harm civilians, they retreat to the village to figure out how to get them to leave. Meanwhile, Anonymus, Dulcia and Mischiefus — a Rome-based family — are denied their apartments due to missing documents. On meeting Obelix and Dogmatix in the forest, the Gauls welcome the family temporarily into their village, and a budding friendship forms between Mischiefus and Obelix.
Asterix, Obelix and Getafix try various schemes to drive the civilians out—heavy rain, boisterous noise, and even a noxious stink—yet the civilians embrace Brittany life, and even begin shopping in the Gauls’ village, sparking a price war for fish and antique weaponry. Frustrated by what the village has become, Asterix, with Getafix and friends, leaves to secure their own apartment at the Mansions, a step allowed by Senator Prospectus.
Caesar then offers free apartments to the Gauls, provoking more tension. To disrupt the exodus, Cacofonix the bard is coaxed to sing at the Mansions, his deafening anthem causing tension, but the rest of the Gauls rally to silence him and move in.
With the village seemingly abandoned, Caesar issues the order to demolish it. The legion refuses to cooperate as Getafix, Mischiefus, and Dogmatix are captured in the Mansions’ lobby, and Obelix collapses from hunger, trapped in a chamber beneath the complex.
Asterix awakens to the siege and the Gauls musters to watch the destruction. They pretend to still possess the magic potion to keep the Romans at bay while Asterix races to the Mansions to free Getafix, Dogmatix, and Mischiefus and resume brewing the potion. But an incomplete potion leaves Asterix in a precarious condition.
Just when all seems lost, Caesar and Senator Prospectus appear to claim victory, only for Obelix to rise from underground, revived by a banquet’s leftovers. The Gauls seize the potion, defeat the Romans in battle, and force the civilians to return to Rome, bringing down the Mansions.
The Gauls celebrate atop the ruins with a grand banquet. Back in Rome, Anonymus and Mischiefus bring a final humiliation to Caesar by tossing Getafix’s instant-growth acorn onto his podium at the Colosseum, sending him dangling from a tree branch.
