Lucky Luke makes the acquaintance of Ma Dalton at Cactus Junction, and he quickly realizes that the saying, 'like mother, like son' has never been so true! Ma is regarded as an eccentric and amusing old lady; for example, she holds up the shopkeepers as she does her shopping. But everything changes when her sons escape from prison and Joe Dalton passes himself off as his mother and starts holding up banks in the area. Lucky Luke, always on the watch, follows the trail of the Dalton family, thanks to Rin tin can...