Asterix and Obelix depart on an adventure to complete twelve impossible tasks to prove to Caesar that they are as strong as the Gods. You'll roar with laughter as they outwit, outrun, and generally outrage the very people who are trying to prove them "only human".

The story opens in a Western saloon, where a young musician with a banjo begins to tell a tale of Lucky Luke and his sworn enemies the Dalton brothers: Joe, William, Jack and Averell. Luke has, once again, as he has done many times before, thrown the four outlaws into jail. The prison is also the abode of a guard dog named Rin Tin Can (Rantanplan in the original French language version).

Julius Caesar plans on kidnapping Getafix but Asterix and Obelix save him. Whilst attempting to scatter the attackers, Obelix accidentally strikes Getafix with a menhir, the impact of which causes amnesia and forgets the recipe of the magic potion.