The Daltons have an uncle who lives in Switzerland, and who is a banker. He wants, as part of an extension policy, to open a bank in the United States. For this, he wants to have his nephews work honestly. Lucky Luke, using the permissions of Governor Sam Pekinpah, takes the Daltons out of prison to try the adventure. So the Daltons try to adapt as bankers, despite the fierce competition of Swindler (swimming financial) and Smallpox (unscrupulous bandit, nor intelligence). 4 against one (plus Lucky Luke), which Dalton will impose his law?