Julius Caesar has invaded Britain and succeeded in his conquest, mainly because the British soldiers under Cassivelaunos stop fighting every day to drink hot water (with a drop of milk) and they refuse to fight over the weekend. 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. 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 Roman galley in the British channel, Obelix mentions the mission, which is reported to the Roman high command in Britain, led by General Motus.
In Britain, the barrel of potion is confiscated from a pub cellar, along with all the barreled "warm beer" (bitter) and Wine 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 labelled with Dipsomaniax but Obelix gets drunk 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. After a stay in the Tower of Londinium, Obelix wakes up and breaks them out of the jail, and they reunite. 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. 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, superstrength.
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.