The plot is that couple of people on Earth were strangely fascinated by the book and couldn't stop reading it until the end. One by one, they all were in a terrible accident that resulted in their deaths, and are reincarnated as the main characters within the book. They followed the script of the story, acting as their character until their memories of their past selves returned. Now, they're in a dilemma: stay in the book and follow the plot, stay in the book yet change things around, or find a way back home.
The villains and heroes already have their reputations constructed by their previous actions. Of course, with their memories and original personalities returning, it would cause them a dilemma. A hero with a sudden personality shift to one of evil, for example.