From what I remember of the story, it is told from several viewpoints of all the characters involved. The basis of the plot is that a bunch of people are in this one store in the mall when suddenly a gunman or gunmen (can’t remember if it was singular or plural) appears and all the people in the store hide in the backroom. One of the characters is a girl wearing a skirt, and she ends up sitting on the clerk’s (or at least an employee) lap. Somehow he ends up telling her about not wanting to die a virgin or something like that and they start having anal (I think it was anal, although it might have started out as normal sex and accidentally turned into anal) sex subtly by covering the act using her skirt. Eventually the other people hiding with them start to notice and it somehow breaks out into an orgy.
I can’t remember if it was all in one giant text or split up into multiple posts/pages.