Flash Sentry's first Royal Bodyguard Duty assignment for the Canterlot Royal Guard was supposed to be an uneventful evening, but when Princess Cadence is kidnapped and carried off by a mysterious look-alike on the night of her engagement party, Flash trails them into the Canterlot Caverns and learns that all of Equestria is in danger! What can a single pegasus who can't even get over his nervousness around royalty possibly do to stop an invasion? This is a Flash Sentry backstory fic, intended to stay as true to canon as possible.