We flash back to Shining Armor consoling a younger Twilight, who had learned that a filly who had befriended her had only done so at the request of her parent, who were trying to get Celestia's ear, and had insulted her when pressed on the subject. Twilight swears off trying to get close to anypony besides him, Spike and Cadance, though Shining tells her to save herself up for ponies who will appreciate her for who she is.
Cut to the present, and Twilight has once again built a wall as she interrogates her imprisoned yet seemingly unconcerned BBBFF, with the ever-present Lancer himself growing concerned at his mistress's new demeanor. Shining's voice claims that Rarity was in on the poisoning and that she had feigned innocence for Twilight's benefit (Twilight having had at least the sense to take her friend's protestations at face value, even as she grows increasingly suspicious and contemptuous of her whole group), and suggests Twilight ask her "about the love letters".
Twilight buggers off, and Lancer demands to know who or what is really in the cell before him- Shining Armor is far too bad a liar to have faked his affection for Cadance all this time as he claims, and he never addressed his sister by her full name as he is now. Careful or we might have a new best pony on our hooves at this rate. He enters the cell and vows to put a stop to this, before belting Shining repeatedly in the hopes of shocking a changeling back into its true form, then (realizing Shining is under a curse) imploring his old student to fight it from the inside. And hearing his lines in Jason Statham's voice seems an even better fit now.
Twilight arrives back at her palace, having taken even more to dark magic use and now bordering on A God Am I mode, and immediately orders her "friends" (gathered at her request) to scram aside from Rarity, privately vowing that they're done growing fat off her own privilege and laying into the protesting Rainbow and Pinkie for their own frivolities. She accuses Rarity of playing her like a piano this whole time and shocks Rarity with her "knowledge" of the love letters, and shrugs off Rarity's protestations of innocence and of believing the notes to be a prank, remembering Rarity's long-ago desire for a prince figure and her and the others' abandonment of her at Cadance's wedding rehearsal. Twilight straight-up banishes her from her sight and from Canterlot, and Rarity barely holds her composure long enough to get out the door. And yet... seeing (and magically scorching) a picture of how happy she and her friends and family were at her coronation is enough to trigger doubt once again....
Rarity goes to board the train back to Ponyville but is waylaid by Princess Celestia, who has in fact paying attention to all of the crap going on. Celestia informs Rarity that Twilight is not fully in control of herself- as Princess Luna was not during her long-ago rebellion- and that Luna has gone to counsel her, but Rarity still believes Twilight will continue to blame her over the letters and heads off.
Twilight attempts to deal with things by reenacting Spider-Man 3, arguing with the inner voice which claims that her friends were holding her back from her true potential and holing up in a bell tower, where Luna finds her and attempts to give her some context. Turns out Luna attempted to tap into King Sombra's magic after he banished the Crystal Empire in order to restore it, but despite her good intentions it played on her desires and insecurities and transformed her into Nightmare Moon, all while she still believed she was in control of herself. Unable to give up on what seems like the only hope of saving Cadance (whom she still seems to think is the only one not to have betrayed her), and swamped with her own dark whisperings against Luna, Twilight.exe stops responding...
...and then she and Luna see her palace is on fire. The Long Dark Night has begun....
