Basically, it's a 'retelling' of the popular "Friendship is Magic" TV series with an OC.
A member of the Musketeer ponies of the royal Guards the ruler of Equestria herself, Princess Celestia has him tasked on protecting one very important unicorn pony; the Princess's personal student, Twilight Sparkle. And that is where the story begins...
Crossover with <a href="http://en.wikipedia.org/wiki/The_Three_Musketeers">The Three Musketeers</a>.