Gaul 49 BC: After a group of Roman legionaries are once again beaten up by the superhuman Gaulish warriors, the soldiers imagine that they can't be human – so they must be gods. Julius Caesar is informed, and gives the Gauls a series of 12 tasks, inspired by the tasks of Hercules.

Chief Vitalstatistix assembles the village’s best warriors, Asterix and Obelix, to do the job. The Roman Caius Tiddlus is sent along with them to guide them and check they complete each task. If the Gauls succeed, Julius Caesar will admit defeat and let them become the Rulers of Rome. But if they should fail, the Gauls will become Caesar's slaves!

The Twelve Tasks of Asterix is based on the Asterix comic book series. René Goscinny and Albert Uderzo, the creators of the series, wrote the story and directed the film themselves; with co-direction by Pierre Watrin and the screenplay co-written by Pierre Tchernia, a friend of Goscinny and Uderzo. The movie directed and produced at Goscinny and Uderzo's own Idefix Studios. It's the first and so far the sole Asterix animated movie based on an original screenplay rather than a published comic book, even though it was later adapted into a comic book as well as an illustrated text story book and a series of twelve books for young readers.