Plot

Asterix and Obelix leave for Britain with a barrel of Getafix's magic potion to help the village of Anticlimax, Asterix's Breton cousin, resist the Roman invasion.

The diminutive Asterix and his rather larger companion Obelix, warriors of the last village in Gaul still free after the Roman invasion, set out on a mission to deliver a barrel of their druid's famous magic potion to help Asterix's cousin in Britain fight off the invading Roman army.—Benjamin John Evans <benj@whitley.unimelb.edu.au>

Julius Caesar has invaded Britain and succeeded in his conquest, mainly because the British soldiers under Cassivelaunos stop fighting every day at 5 PM to drink hot water (with a drop of milk) and they refuse to fight over the weekend. The Roman legions are better trained, and better equipped as compared to the casual British troops. Caesar, using his military genius, decides only to fight when they stop to drink hot water and at weekends. As with Gaul, a single village remains independent, defying the Romans.

One member of the village, Anticlimax, is dispatched to Gaul to enlist the help of Getafix the druid in providing magic potion for the British rebels. Meanwhile, in Gaul, Asterix's village wakes to up yet another normal morning of chaos and everybody fighting everybody else. Obelix is depressed as he hasn't bashed any Roman legions for many months now. They are all busy fighting in Britain. Anticlimax reaches Gaul and meets Asterix. Getafix brews a barrel of the potion for Anticlimax to take back to his village, but Anticlimax cannot even lift the barrel.

It is decided that Asterix (Anticlimax's second cousin twice removed) and Obelix should accompany him back to his village to help transport a barrel of the potion. However, while beating up a pirate galley in the British channel, Obelix saves a Venetian merchant ship. The captain gives Asterix some herbs as thanks. Further along, Obelix beats up a Roman galley, but ends up mentioning their mission in Britain, which is reported to the Roman high command in Britain.

Anticlimax takes Asterix and Obelix to Dipsomaniax's pub for their first British meal, as Obelix was feeling very hungry. The Roman General orders the arrest of 2 Gauls who have entered Britain, but Asterix and Obelix manage to escape to Londinium. The General then orders all the beer in Londinium to be confiscated. Seeing the patrols in Londinium, Anticlimax takes Asterix and Obelix to his good friend Dipsomaniax. Dipsomaniax owns a pub and hides the barrel among his wine in the cellar.

The barrel of potion is confiscated from a pub cellar, along with all the "warm beer" (bitter) throughout Londinium, by the Romans, who set about tasting the barrels to find the right one. Soon the whole unit assigned to the testing is hopelessly drunk. Asterix and Obelix steal all the barrels labeled with Dipsomaniax but Obelix gets drunk trying to figure the barrel with the potion and starts a fight with some passing Roman soldiers. During the fracas, a thief steals the cart with the barrels.

In the meantime, Anticlimax and Asterix leave Obelix at Dipsomaniax's pub to sleep off his hangover. While Anticlimax and Asterix go in search of the thief, the Romans capture the sleeping Obelix and Dipsomaniax, and raze the pub. The Romans were able to track Obelix at the pub since the only barrel's missing from Roman custody were those belonging to Dipsomaniax. After a stay in the Tower of Londonium, Obelix wakes up and breaks them out of the jail, and he and Asterix reunite. The potion barrel was sold by the thief to an unknown buyer. Dogmatix tracks the thief down, who reveals that he sold one barrel from his stolen cart. The three heroes hunt down the potion, which is being used as a pick-me-up for a Rugby team, which ends up mauling their opponents in the match. The trio take the potion via the river but are pursued by a Roman galley. Eventually the potion is lost in the Thames after an attack from a Roman catapult, though it gives some fish, and a fisherman who is pulled in, super-strength.

Finally reaching the independent village, Asterix eases the Britons' disappointment by claiming that he carries herbs to remake the potion, as working for Getafix has given him that knowledge. These are later revealed to be tea. With a psychological boost, the village prevails against the Romans.

Asterix and Obelix return home to the inevitable feast. The Britons like the tea so much, they proclaim it shall be their national drink.