Once again, Luke is called to the penitentiary, though for once the Daltons haven't escaped. They have, however, just learned that their cousin Emmett, last survivor of the original Dalton gang, has a son - and that Averell was chosen as his godfather! Now Lucky Luke has to accompany the dumbest bandits in the West to the young boy's house, as Averell has been temporarily entrusted with his education. A job that his brothers see as an excellent opportunity...

During the darkest days of the Civil War, wicked cutthroats came into possession of six pistols of otherworldly power. In time the Sixth Gun, the most dangerous of the weapons, vanished. When the gun surfaces in the hands of an innocent girl, dark forces reawaken. Vile men thought long dead set their sights on retrieving the gun and killing the girl. Only Drake Sinclair, a gunfighter with a shadowy past, stands in their way.

Transferred to a new penitentiary, the Daltons are put in a cell with Fennimore Buttercup, a counterfeiter who soon begins to regret having such noisy cellmates. To get rid of the annoying brothers, he sends them on the trail of his (made-up) stash: $100,000 buried at the foot of a boulder in Red Rock Junction. One prison escape later, pursued by Lucky Luke, they discover to their horror that the spot they seek is inside another penitentiary!

Passing through New Orleans, Lucky Luke stumbles upon a heated argument between two steamboat captains. The argument soon turns into a wager: whichever boat reaches Minneapolis first after steaming up the Mississippi will win the exclusive rights to the route. Captain Barstow quickly invites Luke to travel on his Daisy Belle, fearing that his opponent will cheat. And while he's not wrong, the biggest danger to both boats remains Old Man River himself...