Plot

Twilight Sparkle is confused by her conflicting feelings when she finds out her brother, Shining Armor, is marrying Princess Celestia's niece, Cadence.

While having a picnic, Spike delivers a message from Princess Celestia to Twilight Sparkle and her friends. They are all being asked to provide assistance and services for an upcoming wedding to take place in Canterlot.

As Twilight wonders just who is getting married, Spike produces another scroll, which is from her brother, Shining Armor, to Princes Mi Amore Cadenza. Naturally, Twilight is quite perturbed about just learning about her brother's upcoming wedding that is just days away.

The group then heads to Canterlot, wherein the kingdom is being protected by a large bubble-dome, and numerous royal guards are stationed all over the kingdom.

Upon meeting up with her brother, Twilight expresses her feelings about not knowing of the marriage, but Shining Armor is surprised that Twilight doesn't know who the Princess really is: Cadance, Twilight's former foal-sitter! This information causes Twilight to suddenly change her mind, as she has nothing but good memories about her former foal-sitter. As well, Shining Armor asks Twilight to be his Best Mare at the wedding, to which she eagerly accepts!

Shortly afterwards, Twilight meets Cadance, but when she attempts to do a fun little dance from their childhood, Cadance seems puzzled at what she is doing.

As the wedding preparations continue, Twilight notices that Cadance seems to be saying one thing to her friends on the preparations, but seeming to be negative about them behind their backs. She brings this to her friend's attention, but they assume she is most likely just overreacting.

Going to visit Shining Armor, Twilight finds Cadance in his residence as well. However, Shining Armor suddenly encounters a sharp pain, and Cadance takes him into a side-room. Peering through a crack in the door, Twilight is shocked when Cadance casts a green-glowing spell from her horn onto his, causing Shining's eyes to roll around. Before the two can return to the main room, she leaves.

The next day at the wedding rehearsal, Twilight is surprised to see that Cadance has gotten rid of her former bridesmaids, and put Twilight's friends in their stead.

However, the rehearsal grinds to a halt when Twilight steps forward, claiming that Cadance is evil. She tells how she has been negative to her friends behind their back, what she saw at Shining Armor's place, and suspects that Cadance has done something to her former bridesmaids.

The accusations send Cadance into tears, and Shining Armor angrily confronts Twilight. He explains that he is in charge of the protective bubble over Canterlot, and the strain gives him severe migraine headaches, of which Cadance has been helping him with. As well, Cadance replaced her bridesmaids, because they simply wanted to be in the wedding party because of the fame it would entail. Shining Armor then claims he wishes Twilight to not be his Best Mare, as everyone clears out of the room, including Princess Celestia.

Twilight is left alone, when suddenly, Cadance appears before her. Twilight claims she's sorry for what she did, when Cadance's eyes suddeny flash green, and a green circle of flames appears around Twilight, pulling her into the floor, as Cadance trots away.