The prison where the Daltons are locked up is full. They will be transferred to another prison near the Mexican border, but the diligence in which they travel will be attacked by Emilio Espuelas and his gang. An alliance between the bandits will be born. The Mexican government, which already has much to do with its own outlaws, does not want the presence of the four bandits on its territory. The American ambassador therefore gives Lucky Luke carte blanche to recover the Daltons.