One possible theme could be the battle between good and evil. If there are real - life monsters in the story, it's likely that there are also men representing the good side trying to overcome them.
Well, without more specific information about this 'Monsters and Men Based on True Story', it's hard to say exactly. It could be a book, a movie, or some other form of media. Maybe it's about real - life events that involved some sort of 'monsters' which could be metaphorical for bad people or real - life dangerous creatures, and how men interacted with them.