The Canterlot Royal Guard are divided in two. The day shift, who patrol the Castle grounds during the day, fighting evil and pushing back great invading forces, and the night shift, who's toughest fight they ever face is the fight to stay awake.
Boring and dull, that's what everypony describes the job as.
However something stirs in the deep tunnels of the Canerlot Castle. Something that doesn't want anypony to wake up.