Arriving at the town of Fort Smith with a prisoner, Lucky Luke is surprised to find a place in turmoil and a thoroughly annoyed sheriff. Belle Starr, a rich ranch owner, keeps posting bail for every bandit arrested in town, with the complicity of a corrupt judge, before recruiting them in her own gang and carrying out various attacks and robberies. Unfortunately, she is also such a pillar of the community that no one believes she could possibly be guilty...