public class Category
{
   public string Name {get;set;}
   public IList<Category> SubCategories {get;set;}
}

public class Story
{
   public string Title {get;set;}
   public IList<Category> Categories {get;set;}
}

dbStory.EnsureIndex(x => x.Categories, "$.Categories[*]");

var stories = dbStory.Find(x => x.Categories.Contains(categoryName));