New York 1855. The Daltons are summoned to court for yet another trial. But the four wretches escape and rob all the banks of the city one after the other. At the end of a crazy chase through the Big Apple, Joe Dalton has barely enough time to hide the loot in the cart of an immigrant caravan before being arrested by Lucky Luke. The members of the caravan, pioneers victim of an unscrupulous cheat, only have 80 days to reach California and reclaim their land. A race against the clock begins. Our solitary cowboy accepts to help the courageous immigrants to brave the hazards of the Wild West, while also accompanying the Daltons to jail. The latter have only one thing on their minds: to get back their loot.