Former Canterlot High School bully and student of Princess Celestia, Sunset Shimmer, who has reformed after being defeated by the magic of Twilight's crown, is ostracized by most of the school despite her efforts to atone. Her only friends are the counterparts of Rainbow Dash, Applejack, Pinkie Pie, Fluttershy and Rarity, who have formed a rock band called "the Rainbooms" to participate in the school's upcoming musical showcase. The five girls discover that the magic left over from Twilight's crown has given them the ability to grow pony-like ears, tails, and wings when they play their instruments.
Hoping to make a fresh impression, Sunset gives a school tour to three new students – Adagio Dazzle, Sonata Dusk, and Aria Blaze – and informs them of the showcase, unaware that they possess magical powers. Calling themselves "the Dazzlings", the trio perform a song that turns the other students aggressive and competitive towards each other, convincing them to turn the showcase into a Battle of the Bands. Sunset and her friends are protected from the song by their own magic, but fail to convince Principal Celestia and Vice Principal Luna of the danger. Sunset remembers a magical book kept by Princess Celestia, her mentor from Equestria, which she uses to send a message requesting Twilight's help.
Receiving Sunset's message, Twilight deduces that the Dazzlings are sirens from Equestria that feed on negative emotions to fuel their singing powers for their goal of world conquest. Twilight uses the book's magic to reactivate the portal, allowing her and Spike to return to the parallel world. Twilight and the girls attempt to lift the Dazzlings' spell, but it takes no effect. Concluding that they must perform a musical counter-spell, the girls enter the Battle of the Bands to give Twilight time to complete the spell. The Rainbooms face hostility and sabotage from the other bands throughout the competition, which raises tensions among Twilight's friends.
Plot Summary from Wikipedia - See more on en.wikipedia.org Text under CC-BY-SA license