Tom Cameron learns that his twin brother is with a group of German spies. They intend to thwart the government's efforts to round up horses for military service.
Drifter Cole Harden is accused of stealing a horse and faces hanging by self-appointed Judge Roy Bean, but Harden manages to talk his way out of it by claiming to be a friend of stage star Lillie Langtry, with whom the judge is obsessed, even though he has never met her. Tensions rise when Harden comes to the defense of a group of struggling homesteaders who Judge Bean is trying to drive away.
The Dalton brothers are to receive their dead uncle's fortune if they eliminate the judge and jury responsible for his execution. And to accomplish this task, they must enlist the help of Lucky Luke.
While on his travel across the Southwestern United States, with his Jeep CJ Renegade and his chestnut colt named Joe Brown, Luke meets Matt, son of a friend of his, Moose, who is in jail. Moose asks Luke to take care of Matt, and to help him to take possession of a piece of land. So starts their travel, full of adventures...