Centurion Crismus Bonus, head of the Roman garrison at the fortified camp of Compendium is very keen on discovering the secret of the Gauls' superhuman strength after four soldiers are knocked out by one man, and sends a spy disguised as a Gaul into the village. The Roman's identity is revealed when he loses his false moustache, but not before he discovers the existence of the magic potion brewed by the Druid Getafix. He also manages to drink the potion after pretending he needs it to get back home as he claims the Romans think he is a spy, and reports his discovery back to the Centurion. Crismus Bonus believes that with this potion, he could overthrow Julius Caesar, and become Emperor himself. So, he and his second-in-command Marcus Ginandtonicus have Getafix captured using a pit in order to get the recipe. He is tortured by having a feather tickle his feet for hours, but does not give in.
Asterix learns of Getafix's capture from a local man, and manages to sneak into the Roman camp where Getafix is being held captive in the man's cart after telling him Compendium has a second-hand cart stall on. He hears Crismus and Ginandtonicus planning to overthrow Caesar using the magic potion. Asterix find Getafix and they concoct a scheme to trouble the Romans. Getafix pretends to agree to the Centurion's ultimatum of making the potion when Asterix pretends to give in to torture, despite the torture not actually having started yet, and demands an unseasonal ingredient like strawberry. While Crismus Bonus' soldiers try to find strawberries, Asterix and Getafix lounge around in comparative luxury, enjoying themselves at the Romans' expense. When the strawberries are bought at a vast sum from a Greek Merchant, the two Gauls eat them, causing anger to Crismus, before Getafix says the potion can be made without strawberries, they just leave a taste in the mouth.
After all the ingredients are found, a potion is prepared that causes the hair and beard of the drinker to grow at a very accelerated pace. The Romans test it on the local man from earlier as Crismus worries about it being poisoned, and when he tests his strength on Asterix, Asterix pretends to be knocked out. The Romans are tricked into drinking this potion and before long, all of them have long hair and beards. They plead with Getafix to make an antidote, who makes a cauldron of vegetable soup (as the effects of the hair potion are about to wear off anyway) and also prepares a small quantity of the real magic potion for Asterix to drink so that they can fight their way out.
As Getafix and Asterix are attempting to escape, they are stopped by a huge army of Roman reinforcements just outside the camp and are captured again. It turns out that Julius Caesar is leading the army and checking on the condition of the area. Upon meeting Asterix and Getafix, Caesar learns of Crismus Bonus' intentions. As punishment, he sends Crismus Bonus and his garrison to Outer Mongolia where there is a barbarian rebellion and frees Asterix and Getafix for giving him the information while reminding them that they are still enemies.