Do you have a join table? You must have a join table named characters_stories with columns named character.id and story.id. I believe that is a requirement for HABTM. In your models, you need to define:
Story
:has_and_belongs_to_many => :characters
Character
:has_and_belongs_to_many => :stories
Once you have it all wired up, you should be able to just do:
char = Character.find(17)
char.stories