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!