Julio César perdonará a un pueblo si los guerreros Asterix y Obelix realizan 12 tareas encomendadas.

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.