Celestia asks Luna to teach Twilight dark magic. Twilight must pass Luna's class in order to graduate. As Luna develops feelings for her student, Twilight questions which princess she really wants to serve and protect.
(Twilight X Luna)

After Nightmare Night, Luna invites Twilight to tea, and they start spending more time together. Princess Celestia teases Luna about it sometimes.

Twilight Sparkle is called to Canterlot Castle where something has happened to Princess Luna that has reverted her to a weakened state, she has to live with Twilight whilst she recuperates her magic.

The next Nightmare Night is approaching fast, and this time Luna's going to come to scare everypony as Nightmare Moon the first time intentionally. A few days before she leaves for Ponyville though, she discovers the friendship reports Twilight has been sending to Celestia. Interested in these, she reads them and realizes she has a crush on Twilight Sparkle.

How will she react to this revelation, and will Twilight return those feelings?

Cover image is by RyuRedwings. Editing done by Destinae Spring on chapters 1 and 2.