Lovers have their reasons. To Twilight Sparkle, the reasons she loves Celestia are perfectly obvious. And to Princess Celestia the reasons why she can't love Twilight back are equally obvious.
It has been over 100 years since Celestia and Twilight's relationship collapsed, since they've seen or spoken to each other. But when Twilight appears in the royal palace, that century of silence will come to an end. Very, very politely.
To keep her job at the library, Twilight Sparkle must face her greatest challege, one which will push her very sanity to the breaking point. No, not the Cosmic Horror stirring in the dreamvoid between universes, worse: Library Patrons.