Caesar has decided that if the Gaulish village cannot be conquered, it will be absorbed into Roman society, and he has commissioned the architect Squareonthehypotenus to design a Roman city around the village, a city to be called "The Mansions of the Gods". He is sent to the fortified camp of Aquarium near the Gaulish village.
Squareonthehypotenus' project runs into difficulties from day one, as he has several close encounters with Asterix, Obelix, and Dogmatix as they hunt for boar while the architect is surveying the forests around the village. He decides the work must be done at night, and tells the international assortment of slaves he has rounded up (including Iberians, Lusitanians, Numidians, Goths, and Belgians) that they must be quiet so as not to disturb the Gauls in the village. They try to get out of working by singing loudly and claiming to be unable to work without doing so, but the architect finally snaps and orders an end to the singing. They only pull down a single tree before a group of Numidians imitate a rooster, and the construction crew members quickly disperse as the Gauls in the village begin to go about their daily business with hours to go before sunrise.
The following day, Asterix, Obelix, and Dogmatix find the fallen tree, and Obelix promptly replants it. The construction crew are ordered to remove the trees after pulling them down, so Getafix treats acorns with a potion that causes them to grow into oak trees instantaneously and has Asterix and Obelix replant the uprooted trees. After several days of this, the increasingly frayed Squareonthehypotenus vows to work the slaves to death if necessary; Asterix overhears this remark, and gives the slaves magic potion to fight back against their employers.
However, rather than overthrow their employers, the slaves simply demand better working conditions, fair pay, and freedom after the first building is completed in exchange for an end to the violence. The legionaries of Aquarium promptly go on strike when they realise they are now getting paid less than the slaves. The slaves' new working conditions do not stop Asterix and Obelix from replanting the trees as quickly as they are pulled down, so the leader of the slaves, the Numidian Flaturtha, finally implores the Gauls to let them finish building the first block of flats so that they can be freed. They agree, and construction is completed, to the disgust of the villagers; Geriatrix says he can't think what's stopping him from knocking the flats down.
The first wave of residents of the Mansions of the Gods arrives from Rome, including a couple who won a flat in the building in a competition (the husband only agreed to the prize when told his alternative was to be thrown to the lions). As construction on the local shops has not yet begun, the residents are invited to shop at the local Roman camps, but some of them decide instead to begin shopping at the Gaulish village, buying fish from Unhygienix and weapons (presented as antiques) from Fulliautomatix's forge. The village is soon overrun by Romans showing interest in buying anything that isn't nailed down, including Getafix's cauldron and Vitalstatistix's shield, and the villagers begin opening antique shops and fish stalls en masse to get a share of the money now flowing into the village.
Asterix, Obelix, and Getafix decide to put an end to the madness. After establishing that there are no vacancies in the Mansions of the Gods, Asterix has Obelix pretend to attack the couple who won their flat in a competition in a quasi-feral rage. The husband finally snaps and he and his wife move out of the block of flats; the newly vacant flat is immediately rented by Asterix and Obelix for Cacofonix, the bard. After just one night of listening to Cacofonix singing, every other resident of the building moves out in disgust.
His plans collapsing around his ears, the furious Squareonthehypotenus invites the legionaries at Aquarium to move into the building until permanent new tenants can be found, but he first forcibly evicts Cacofonix, who returns to the village to find the fishmongers and antiques salesmen all going out of business. However, just as the villagers prepare to castigate the bard for causing the collapse of their trade with the Romans, Asterix draws their attention instead to the grave "insult" of the Romans kicking one of their fellow villagers out of his home. They load up on magic potion and descend on the block of flats and its residents, bashing in the legionaries and reducing the building to rubble.
The battered Squareonthehypotenus announces his plans to go to Egypt and build pyramids, while Asterix and Obelix replant the trees among the ruins of the Mansions of the Gods. The book ends, as usual, with a celebratory banquet, and even Cacofonix is allowed to take part in honour of his role in driving out the Romans.
- Attack on One Is an Attack on All: Cacofonix being evicted from the Mansion serves as the Gauls' casus belli. Even better, Asterix and Getafix actually didn't count on that happening - they only wanted Cacofonix to drive away the tenants non-violently with his terrible singing.
- Author Tract: René Goscinny was highly critical of the direction urban planning had taken following the May 1968 riots, and made it a target of satire in The Mansions of the Gods.
- Damned by Faint Praise: When Caesar is outlining Squareonthehypotenus' credentials as architect of the Mansions of the Gods, he mentions that he has designed many insulae (blocks of flats) - "Some of which have not fallen down." (Which may foreshadow the eventual fate of the block of flats built outside the Gaulish village.) This is in fact an historically accurate joke: the City of Rome was full of cheap flat blocks which collapsed on a regular basis.
- Every Episode Ending: Subverted; in recognition of his role in saving the day, Cacofonix is allowed to take part in the banquet at the end.
- Everyone Has Standards: The villagers might not particularly like Cacofonix, but the minute they discover that the Romans have evicted him from his apartment — something that Asterix and Getafix hadn't actually planned on happening, with their only goal for him being to scare away the Roman tenants — they immediately march on the Mansions of the Gods with the express goal of totally destroying the building.
- Lazy Bum: The Lusitanian slave. The Romans set about building the flats in secret at night, but the Nubians establish that they can't work without singing, so Squareonthehypotenus excuses them from work. Gradually, other groups of slaves also make it clear that they can't work without singing either, so they're all excused work. Then the Lusitanian ambles over with a hefty dose of either Obfuscating Stupidity, Insane Troll Logic, or both:Lusitanian: 'Scuse me, I'm Lusitanian. I can't sing, but I could do you a recitation if you like.
- Mass "Oh, Crap!": On Cacofonix's first night in his new flat in the Mansions of the Gods:Cacofonix: ("singing") J'aime les forêts/Dirladada/Parce qu'il y a des sangliers/Dirladada... (the other tenants all freak out)
- Self-Serving Memory: Caesar's memory of Vercingetorix's surrender has the Gaulish chieftain kneeling pitiably at his feet as the Roman general looks on impassively. The series' "official" version of these events, as seen in Asterix the Gaul and Asterix and the Chieftain's Shield, has Vercingetorix taking the order to throw his arms at Caesar's feet literally, causing Caesar to hobble off in agony.
- Shout-Out: The master of ceremonies at the competition in the Circus Maximus for a flat in the Mansions of the Gods is a caricature of French television presenter Guy Lux, the creator and original host of the game shows Intervilles and its Spiritual Successor Jeux sans frontières (known across the Channel as It's a Knockout).
- Slave Liberation: Subverted. Rather than rising against the Romans, the potion-powered slaves simply demand rights, such as wages and freedom when their task is accomplished (and one even asks an overseer to whip him, he's feeling a bit tired).
- Surprisingly Realistic Outcome: Getafix gives the potion to the Roman slaves so they'd rise up against their masters. However, in Ancient Rome slaves were treated as persons and were even paid a small stipend so they could eventually buy back their freedom (so their masters wouldn't have to pay for their upkeep in their old age) and the Romans executed escaped slaves, so the slaves instead demand an increase of said stipend and to be freed upon completion, and the Romans comply.
- Third-Person Person: Caesar has a brief moment of only referring to himself in the third person (as the real Caesar was prone to doing) during his presentation of the Mansions of the Gods project.
- This Is What the Building Will Look Like: It's a given that the Mansions of the Gods project will never come to fruition, so the first few pages involve Caesar unveiling a model of the city to show how it would look if it were completed.
- Throw the Dog a Bone: Cacofonix, in recognition of his role in destroying the flats and driving out the Romans, is allowed to take part in the banquet at the end.
- Unwitting Pawn:
- The first boarders for the Mansions of the Gods. They don't know that the whole project is a scheme of Caesar's to subdue the Gaulish Village, and Getafix refuses to give out Magic Potion against them for this reason.Roman: What if I refuse to go to Gaul?
Announcer: You'll stay in the arena and we'll let in the lions.
Roman: Then in that case, I accept.
- Cacofonix. He had no idea when Asterix, Getafix and Obelix arranged for him to move into the one couple's flat, that he was being used to drive out the other tenants with his singing. And while it may not have been in his friends' plan to have him evicted to accommodate the Roman legion moving into the flats, it certainly worked in their favor.
- The first boarders for the Mansions of the Gods. They don't know that the whole project is a scheme of Caesar's to subdue the Gaulish Village, and Getafix refuses to give out Magic Potion against them for this reason.