A couple weeks before the last chapter, the Royal Family (and the Element Bearers in full regalia for some reason) met with Karkat (accompanied by Aristotle) in the Crystal Empire Republic to begin negotiating the treaty that was mentioned last chapter. The first night there, a unicorn Aristotle didn't know broke into his room, apparently looking for something. Aristotle thought her mane was nice, and he said so, which caused the unicorn to blush. Sunset Shimmer, the mystery thief, still managed to steal Twilight's Crown of Girliness and escape into a magic mirror. Aristotle and Karkat went after her, because such an incursion could not go unpunished.
After taking stock of their surroundings and surveilling their targets, the 2 men agreed that Aristotle would, on the following day (they had been informed of the time limit on the portal and made Majora's Mask jokes), cause a distraction, and Karkat would go and actually steal back the Diadem of Femininity. Aristotle wasn't very good at things like "stealth" or "subtlety" or "knowing when to shut up".
The next day started off as a fairly normal one for Canterlot High: Sunset Shimmer keeping the people down, excitement for the upcoming Prom/Dance/Whatever, that sort of thing. But at lunch, things took a turn for the, uh, weird. Yeah, let's go with that. A man crashed through the ceiling, shouting,"MR. SENTRY! I am...I forgot my name, but I am the first representative of the League of Sunset Shimmer's Heroic Ex-Boyfriends! To date her, you must fight and defeat all of us. What say you?"
Flash(Woah-Oh! Savior of the Universe!) Sentry informed the obvious lunatic that he had recently broken up with Sunset Shimmer, promoting an offer to join the League. The application was held immediately, and the special skills demonstration ended in the crazy guy and Flash(Woah-Oh! He'll save every one of us!) Sentry having what could only be described as a sword fight with guitars instead of swords. This lasted most of the lunch period, with the prospective Leaguer informed that he would receive a letter within a business week if he was accepted. Meanwhile, Karkat stole the Precious Thing.
As their surveilling had taken a couple days, the Super Human Brothers only managed to beat their deadline back into the portal by a couple of hours. Sunset Shimmer was once more trapped in a world she never made, and Twilight had her Tiara of Womanhood back. Somehow, the Crystal Empire Republic hadn't lost any face from the incident, and good times were had by our (debatably) intrepid heroes.