Canterlot High is hosting a musical showcase and Rainbow Dash, Applejack, Pinkie Pie, Rarity, and Fluttershy are going to be performing with their new band, The Rainbooms. But when a mysterious new girl group, The Dazzlings, arrive at CHS, the music showcase is suddenly turned into a Battle of the Bands. As tensions mount between the competing musical acts, a reformed Sunset Shimmer begins to suspect that these new girls are actually magical beings from Equestria and are after something far more sinister than being named best band at Canterlot High. The Rainbooms turn to Princess Twilight Sparkle to return to their world and help them perform a musical counter-spell that can defeat these new villains. But as Princess Twilight struggles to compose the magical song that will save CHS, trouble starts brewing between the members of the Rainbooms and it falls on Sunset Shimmer to help the Rainbooms resolve their differences in time to win the most epic musical battle of them all.