This is essentially a tale of how Flash Sentry became the pony we've always seen, and what kind of pony he truly is. Why and how did he become a guard? Who are his friends? What experiences did he suffer?
Side note: The Mane 6 will make small cameos and there will be some references to them from other characters, but they won't play a major part of the story. This is Flash's story, not his and Twilight's supposed story.