Three mysterious girls show up at canterlot high and cause trouble. With the magic of friendship, they want to defeat the mysterious girls.
Music rules and rainbows rock as Twilight Sparkle and pals compete for the top spot in the Canterlot High "Mane Event" talent show. The girls must rock their way to the top, and outshine rival Adagio Dazzle and her band The Dazzlings, to restore harmony back to Canterlot High.
Three mysterious girls wearing gem necklaces show up to Canterlot High School intent on controlling everyone in that world. The girls in the band, The Rainbooms, need Twilight Sparkle to return from Equestria in order to overcome this new threat.
- The film starts in a small cafe,where a number of teenagers are seen arguing. It is soon apparent that their bad moods are being created by the siren song of three teenage girls in a corner booth. The three girls bicker that the negative energy they've absorbed isn't enough to properly 'feed' them, when a bright flash outside attracts their attention (the magical epilogue of the first Equestria Girls film!). The leader of the girls, Adagio Dazzle, recognizes the effects as "Equestrian Magic," and decides along with her cohorts (Sonata Dusk and Aria Blaze), to find its source, and regain their former powers.
The scene then cuts to some time afterwards, with Canterlot High School preparing for a school-wide Musical Showcase. Since the events of the first film, Sunset Shimmer has been guided by Applejack, Fluttershy, Pinkie Pie, Rainbow Dash, and Rarity in helping to reform herself from her old ways. However, much of the school has not so easily forgiven her past transgressions.
As the girls prepare for the Showcase (with Sunset watching), their pony tail and ears manifest while playing, a side-effect that is unexplained.
Soon after, Sunset goes to welcome three new students to the school and show them around. The three girls happen to be Adagio, Sonata, and Aria. Sunset is a little surprised by their demeanor, as well as the red pendants around their necks.
After the tour, Sunset tells her friends that she fees there was something 'off' about the girls. As they talk, the three strut into the cafeteria, singing a song about having a 'Battle of the Bands,' that soon has almost everyone in the cafeteria determined to be the best musical act at the school. However, the song doesn't affect Sunset or the 'Mane 5.'
The girls are sure there is something evil about the girls, but going to Principal Celestia and Vice-Principal Luna proves ineffective, as the three girls have already gotten to them. Celestia tells how the girls have convinced them to turn the Musical Showcase into a Battle of the Bands, and the three have already signed up (under the name, The Dazzlings).
With noone else to turn to, the girls first thought is to try and contact Twilight Sparkle, but the portal between their worlds is still closed. It is then that Sunset Shimmer remembers a journal she used to keep when she was Princess Celestia's pupil. When she would write in the book, what she wrote would appear in a book in the Canterlot Library.
The message gets through to the other book, which happens to be arriving in a delivery to Twilight's new castle. Twilight reads through the book, and determines that the girls her Canterlot High friends are talking about, are a trio of creatures known as "The Sirens."
According to a book in her new library, Twilight finds that the Sirens were 3 creatures that fed on the negative energy of other ponies, but were banished from Equestria by Starswirl the Bearded.
Twilight wants to help her other-world friends, but it soon becomes apparent that the main portal is still closed. However, realizing that the book that Sunset Shimmer has written in has cross-dimensional properties, Twilight rigs a special portal, that uses the magic in the journal, to create a temporary portal that she and Spike can enter through.
Twilight's friends are of course happy to see her, with Spike helping to fill them in on what has happened since Twilight was last there (such as her title as a "Princess of Friendship," and her new castle).
Hoping to stop the Dazzlings, the girls go to a party that evening where all the acts (and the Dazzlings) will appear. However, the three girls' powers have everyone at odds with each other, and noone is willing to mingle outside of their performing group. Twilight and her friends (minus Sunset Shimmer) then confront the Dazzlings. Holding hands and chanting "Friendship is Magic!," leads to an embarrassing situation, when nothing happens!
Adagio Dazzle takes the opportunity to psyche up the rest of the groups, claiming that 'these girls' seem eager to win.' However, as Twilight and her friends leave, Adagio makes note that 'these girls' are special, in that they are not under the Dazzling's negative magic effects.
Outside, Twilight wonders just why her plan didn't work. Sunset Shimmer helps surmise that the Dazzlings' power comes from their music, and the girls decide that the best way to counter it, would be to make a musical counter-spell.
Twilight reluctantly volunteers to create one, but after a full day of work, hasn't come up with anything in time for the auditions for the Battle of the Bands. Since they don't have a counter-spell ready, the girls plan to just keep active in the auditions, figuring if they make it to the finals, that should give Twilight enough time.
At the auditions, the girls' song attempts go awry due to some sabotaging by several of the students, as well as each of the girls having their own wants and issues. Hoping to calm the girls down, Sunset Shimmer requests they go practice elsewhere, while she goes to keep an eye on the Dazzlings.
However, the Dazzlings find her, and dig into Sunset's ego, claiming they know of her past, and that she'll be even more forgotten very soon.
As the auditions continue, numerous groups are eliminated, until it's down to the Dazzlings, the Rainbooms, and (The Great and Powerful) Trixie.
To try and push them into the finals, Rainbow Dash recommends they do her song, "Awesome As I Want To Be." However, getting too into her song, Rainbow begins to materialize her pony ears, causing Sunset to panic and tackle her, throwing the group into chaos, and the school to laugh at her 'bad girl' efforts.
The girls are sure they're done, but they (and Trixie) are shocked when they are confirmed to go up against the Dazzlings in the finals.
The girls next are seen on the stage rehearsing, when Trixie shows up, and still upset at losing to the girls, sets off a trap door, trapping the Rainbooms under the stage. With the girls no-shows, Trixie performs on stage, before the Dazzlings go on.
However, in their trapped quarters, the Rainbooms begin to argue amongst themselves, over all the little things that have been problems during their time as a band. Sunset sees this, and pleads with them to stop, and that the negative energy they are giving off, has got to be what the Dazzlings hoped for all along!
The girls stop their arguing, and manage to escape with the help of Spike and DJ PonE.