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). No sooner have the Daltons entered the jail than they are met by a lawyer named Augustus Betting. Betting informs the brothers that their Uncle Henry Dalton has died by hanging. However, over the course of his criminal career, Henry Dalton amassed quite a fortune, and has chosen to leave it all to his nephews on the condition that they kill the judge and jury who sentenced him to death. To make sure that the task is completed, Henry Dalton states in his will that his nephews must be accompanied by the only honest man that he has ever known, Lucky Luke. If the task is not successfully completed, the entire fortune will instead be given to charity. The brothers then decide to tunnel out of the jail, but end up digging into the dynamite storage building. When Averell lights a match, the building blows