Before Twilight and the return of both Princess Luna and the elements of harmony there was just Princess Celestia and her niece Cadence, when a changeling spy approaches the princess speaking of an upcoming attack on Canterlot she has no choice but to plan a defence, but her royal guard aren't trained to fight off Changelings, in fact Equestria hadn't seen changelings for over a hundred years.

The princess decides that now is the time to unleash her secret weapon, eight hired mercenaries that she had collected from around the know lands of Equestria and beyond, plus the new changeling Spy. Hopefully they won't fail in their first real mission.

((This is pretty much what would happen if the tf2 mercs were born in the mlp universe, so things are probably going to be a little different about them, you have been warned. Takes place a few years before the return of Nightmare Moon))
((This story is not related to my other tf2 x mlp crossovers, it's just an idea that I had))