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

Asterix, el pequeño guerrero, su compañero Obelix y el perro Idefix se aventuran en la Roma antigua.

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).