One afternoon when Twilight Sparkle and her friends were having a picnic, Spike showed up, gasping for air, with a message from Princess Celestia. The letter talked about a wedding being held in Canterlot whose preparations required all of the skills of Twilight and her friends. Fluttershy was tasked with providing the music for the wedding, Pinkie Pie would arrange the reception, Applejack would provide the food, Rainbow Dash would perform a sonic rainboom after the bride and groom say their I-dos, Rarity would be in charge of designing the dresses, and Twilight would make sure everything went according to plan.
All of the ponies were rather excited about their roles in the wedding, but they still had no idea who was getting married. Spike realized that he should have given Twilight the other note he received first that announced that Princess Mi Amore Cadenza and Twilight's big brother, Shining Armor, were the bride and groom.
Twilight was rather upset by this news. If her brother was getting married, she expected to hear about it before the invitations were sent out. The upset pony explained to her friends that before she moved to Ponyville, Shining Armor was her only good friend. Twilight also mentioned that Shining Armor was Captain of the Royal Guard, which got her friends even more interested in the wedding.
The purple unicorn was still depressed as she and her friends rode the train into Canterlot. Applejack tried to reassure Twilight that regardless of Shining Armor's marital status, he would always have time for her. Still, Twilight was skeptical.
The train had to pass through a giant magical bubble surrounding the entire city, which the ponies found a little curious. Upon arriving at the station, the pony friends also found the city crawling with guards, likely there to ensure the wedding went as planned.
Twilight was especially eager to disembark from the train because she felt the need to give her brother a piece of her mind. After finding him attending to his guards, she chastised him for not announcing his marriage plans to her in person. Shining Armor explained that he had been busy attending to security issues. A threat had been made against Canterlot and keeping the city safe required whatever attention he could not spare for his wedding. However, Shining Armor cheered Twilight up a bit by asking her to be his best mare.
Twilight was thrilled to have such a prestigious place in the wedding party. She accepted the importance of his role as royal guard, but she was still upset about not knowing who the bride was. However, Shining Armor revealed that she did know. Princess Mi Amore Cadenza was actually Cadance, Twilight's old foalsitter. This immediately cheered the purple unicorn up. Twilight loved spending time with Cadance.
Shortly after Twilight realized who she was, Cadance met up with the two pony siblings. However, she responded coldly to the old rhyme they used to say together and acted as if she did not even know who Twilight was.
Princess Cadance's sour behaviour continued when she arrived to check out the food preparation Applejack was in charge of. Her apple fritters were delicious enough to cheer up Twilight after Cadance's cold reception, but they were not enough to impress the princess. Cadance also was not a fan of the wedding dresses Rarity had prepared, requesting more beading and a longer train for her own dress and a colour change for the bridesmaids' dresses. She also criticized Pinkie Pie's reception plans, saying that they would be perfect for a six-year-old's birthday party. (Pinkie took this as a compliment.)
As night fell, Twilight met up with her friends and complained about Cadance's rude behaviour. However, her friends were far more understanding than she was, chalking up the princess' bad attitude to nerves and a want to have her wedding day be perfect. Twilight was still convinced that Cadance did not deserve to be the bride of her brother and stormed off when her friends did not agree.
She went to go talk to her brother about her concerns about his bride. Before she could voice her opinion, Twilight was interrupted by Cadance walking in on their conversation. She too wanted to talk to Shining Armor in private, so the stallion went into the kitchen to hear what she had to say. Twilight was curious and listened in on their conversation. It seemed to be a simple argument about a small item of clothing Shining Armor wanted to wear to the wedding. However, when Shining Armor started reacting to a headache he had, Cadance used some kind of sinister-looking spell on him. Twilight was convinced this was some kind of evil magic, and quickly ran out of the house before Shining Armor could ask what Twilight wanted to talk about.
She ran to tell her friends about what she had seen, but the other five ponies had recently been honoured with the privilege of being Cadance's new bridesmaids. The princess did not say what happened to the old bridesmaids, but Twilight's friends were too excited to care. The purple unicorn realized that her friends now thought too highly of Cadance to believe her accusations that the princess was evil.
However, that did not stop Twilight from voicing her opinion at the wedding rehearsal. Her friends tried to prevent her from embarrassing herself, but Twilight made it quite clear she needed to explain how evil Cadance was. She mentioned the spell Cadance cast on Shining Armor, her missing bridesmaids, and her rude behaviour. Princess Cadance stormed out of the room crying. For a brief moment, Twilight felt as if she had accomplished something.
She realized that she may have stepped over the line when her brother confronted her. Furious, he explained that the "evil" spell Cadance cast on him was actually something to help with the headaches he was getting while keeping the city safe with his protective bubble spell. Shining Armor also explained that Cadance replaced her old bridesmaids when she realized that they were only interested in meeting up with Canterlot royalty. Finally, he revealed that Cadance was so rude to everyone because she was stressed out from doing all of the wedding planning herself. Shining Armor was so busy keeping the city safe that he had no spare time to help Cadance. After Shining Armor defended his bride, he announced that Twilight would no longer be his Best Mare. He also told her that she might as well skip showing up to the wedding.
Shining Armor went off to go comfort his bride, shortly followed by Twilight's friends as well. Even Celestia was not in the mood to entertain Twilight's accusations. Soon the unicorn was left in the wedding hall alone to think about what she had done. She was distraught that she had been so judgmental. Instead of gaining a sister in the form of Princess Cadance, it looked as if she had lost her brother.
Cadance showed up after everyone else had left and allowed Twilight to say that she was sorry. However, Cadance no longer seemed upset and informed Twilight that she would be sorry, shortly before engulfing her in a sphere of green flame. As the flaming bubble drifted down beneath the floor, Cadance walked away with a sinister grin.