While it bears the name of The Ysabel Kid, Loncey Dalton Ysabel, this is not a solo adventure. It's actually the first grouping of The Floating Outfit principles, The Kid, Dusty Fog and ultimately Mark Counter.
The story begins when emissaries from President Ulysses S. Grant call on Ole Devil Hardin to request his help in bringing a ex-Confederate Bushrod Shelton back from a campaign in Mexico. Shelton is fighting on behalf of French forces, and the U.S. government would rather back Mexican troops in order to avoid a European-backed stronghold on its Southern border.
Since Hardin is unable to go himself he sends his nephew, Dusty Fog, with a letter of introduction. Ex-Confederate captain Fog has not travelled far before encountering the half-Comanche smuggler known as The Ysabel Kid. The two in turn encounter a rifle salesman and decide accompanying him South will help fulfill their mission by buying Shelton a free path back to the United States.
Their journey with the mule train carrying repeater rifles makes for exciting reading. Along the way The Kid is able to track down the men who killed his father. One is encountered in a showdown as exciting as a Louis L'Amour conclusion that hinges on details about rifles of the era.
Mark Counter is brought into the fold while in Mexico, and by the novel's end Dusty and company are headed back to Ole Devil's place to form The Floating Outfit.