Overwatch and Steel Blade are dedicated guards, but even guards can get distracted. When Steel Blade can't get the thought of a certain charge of his out of his mind, Overwatch helps him focus again.

This was inspired by the Twilyverse. Reading anything from that isn't necessary, but it might help if you've read those already. Celestia's Tiny Student was written by CommanderX5, and can be found here: http://www.fimfiction.net/story/108209/celestias-tiny-student