Disaster strikes in the Gaulish village when Getafix the druid breaks his golden sickle. Without one, he cannot attend the annual conference of druids, or cut mistletoe for the magic potion which keeps the Roman armies at bay. Asterix and his friend Obelix set out for Lutetia (present-day Paris) to buy a new one from Obelix's cousin, the skilled sicklesmith Metallurgix. On the way, they encounter bandits, but easily defeat them. A man in an Inn they stay at says sickles are in short supply in Lutetia. They finally get to the city.
However, Metallurgix has mysteriously gone missing, and before long, our heroes are exploring the underworld of the big city. They go to an Inn opposite, but when they ask about Metallurgix, the man close up, and after they are gone, gives a description of them to another man. This man finds them, and introduces himself as Clovogarlix, claiming to be a friend of Metallurgix. Saying that Metallurgix has retired from sickle making, he takes them to a place where they meet a man called Navishtrix who tries to sell them a sickle for a vast price, though they refuse and wreck the place. They are arrested, but released by the bored Prefect Surplus Dairyprodus, and find out from a Centurion that Metallurgix may have been kidnapped. They uncover a sickle-trafficking gang with sponsors high up the Roman bureaucracy, whose shadowy business is run from below a portal dolmen in the Boulogne forest. The two enter the forest and get directions from a bandit they save from wolves. They get into the lair and find a hoard of Golden Sickles, but are attacked by a gang that includes Clovogarlix and Navishtrix, who the Gauls defeat. Navishtrix gets away, but they find him with Surplus, revealed to be the leader. When all is revealed that Surplus did it all just for mere amusement, the Gauls rescue Metallurgix, and Surplus is arrested for sickle-trafficking, while the sickles beneath the Dolmen have been returned to Metallurgix, they can go home with a new sickle for the druid which Metallurgix gives them for free and the magic potions will continue to flow.