Britain has been invaded by the Romans and soon all of Britain is occupied... well not all, one small village still holds out against the might of the Roman Empire. The inhabitants of this village seek help from the Gauls (as one of them is a cousin to Asterix) and pretty soon Asterix and Obelix are on their way to Britain with a barrel of magic potion. Unfortunately the Romans find out about the barrel and confiscate every barrel in Britain.
After overcoming the Romans, a barrel thief and the local football team the Gauls manage to recover the barrel only to have it smashed by a Roman catapult. Asterix using all of his ingenuity manages to convince the Britains that he can make more of the magic potion. Thus using a few strange leaves that Getafix had given him he brews this 'magic potion' and the Britains manage to defeat the Romans.
Asterix's next adventure is 'Asterix and Cleopatra'.