Twilight is called to Canterlot to help with an emergency of magical nature. Princess Luna is trapped in a magical construct of her own making. The problem being that no one can reach her and she is somehow unable to free herself. As Twilight soon discovers the only way to help the princess is to change her own magical frequency to Luna's, and entering Luna's dreamscape and helping push back the resurfacing Nightmare Moon. The battle Leaves Luna in an extremely weakened state causing her to loose her power to control the moon until she recovers in a few years with the help of Twilight's shared magic. The two must also deal with the threat posed by Nightmare Moon's return.
This is based off of arandompenguin's Oldnew Luna with his permission but will diverge from the story he made pretty early on.