From publisher blurb:
Have you ever wanted more story in your roleplaying game stories?
In a dark horse story, the protagonist and antagonist are not evenly matched. The odds are stacked heavily in favor of the antagonist. Initially no one takes the protagonist seriously, but over the course of the story they shake off their underdog status to become successful and respected.
Examples of the dark horse plot include Cinderella, Forrest Gump, The Hunger Games, One Flew Over the Cuckoo’s Nest, and Rocky.