I'm not sure specifically what the '5th quarter true story' is about as there could be many stories with this name. It might be related to an extra period in a game (like in sports, though usually there are only four quarters), or it could be a unique story about a particular community or event.
It could be about the unexpected. In a normal scenario, there are only four quarters. So the '5th quarter' true story might be centered around something that no one anticipated. Maybe it's a story of an underdog rising to the occasion in this unforeseen 'extra' time. This would make the theme about the power of the unexpected and how it can change the course of events.
The 'the 5th quarter true story' could potentially be about an event that occurred after the normal four quarters of a game or activity. Maybe it's about a come - back or an extraordinary event that took place in what is metaphorically called the '5th quarter'. Without more context, it's hard to be more specific.