Plot Summary: Spoiler Warning!
Equesria Girls is back, albeit in a kind of short sequel. This time, a trio of horse-sirens (?) have been banished to the human realm. Apparently, Celestia applied a NIMBY (not in my backyard) approach to the sirens, who are known for causing disharmony, which makes people and ponies more susceptible to being controlled by their magical singing. The three girls' powers are stored in red gemstones they wear on black chokers, in what I assume is a product tie-in with Hot Topic.
The girls' leader creates a plan to sew discord and hatred at Canterlot High, which was still riding a "positive vibes" wave from the events of Equestria Girls. Former villainess Sunset Shimmer, however, is having trouble fitting in, seeing as how she basically kind of turned into a demon and tried to enslave the school. Hey, I've had some rough periods too, cut that girl some slack!
So the sirens/ walking Bratz dolls decide to pose as students (of course!) and infiltrate the popularity pyramid of the high school, in order to feed off their discontent. They were also led to Canterlot High by the events of the ending of Equestria Girls, because they sensed the strong force of the Equestrian magic Twilight Sparkle used to defeat Sunset Shimmer.
What they end up doing is creating a "Battle of the Bands" event to replace the upcoming music festival. By using their siren singing, they convince everyone to hate everyone else and to see the Battle as a high-stakes, winner-take-all competition.
Since the girls, the "mane 5" plus Sunset Shimmer, realize something's up with these new girls, Sunset Shimmer finds a magical way to contact Twilight Sparkle. Twilight, in what seems like a huge Ass Pull, builds her own portal to the human world.
But, winning Battle of the Bands and using a singing counterspell against the sirens, Twilight's plan, proves to be a challenge. The band, made up of the mane 5 and Twilight, can't seem to go too long without some kind of bickering. Then in the finals, they get locked in a pit by a determined TRIXIE!, who is sure that she's going to be number one.
However, it's really a pep talk on friendship delivered by Sunset Shimmer that saves the day this time, and with that they're able to get their adorable doll-like asses in gear and, with a little help from DJ Pon3/Vinyl Scratch, they are able to put on an awesome concert, which works up the magic of friendship enough that they can defeat the sirens, and Twilight can return back to Equestria.
I would classify this as, better than I expected, and basically as good as Equestria Girls. Meaning, if you seriously disliked Equestria Girls, you're also going to hate this movie, because it is a sequel and has a similar plot. However, Equestria Girls actually grew on me over time, and this was a good sequel in many ways.
For starters, despite the short run time (72 minutes), it manages to get in a lot of My Little Pony references for the fans. It also gives a bigger role to fan-favorite minor characters such as Starswirl the Bearded (who we see was originally responsible for defeating the sirens years ago), Trixie, and Vinyl Scratch.
Like Equestria Girls, I think it's good, clean, classy fun for preteen to teen girls. I'm not sure if it has the extra-demographic appeal that My Little Pony; Friendship is Magic itself has had, but it has everything that I think will make it appealing to what I'm guessing is the target audience, 11-15 year-old girls.
One thing the show suffers from here is too many songs that all sound kind of the same. It feels like "Soundtrack: The Musical" too often, and all of it is the same, Disney-pop sound, and while they manage to get a decent song or too in there, the main theme.
The plot also makes little sense/ relies heavily on contrivances, and the plot and lesson of this movie are basically exact repeats of the previous Equestria Girls'. Another thing repeated is the groan-worthy contrived romantic interludes with Twilight and Flash Sentry. It feels more like an episode of a new series than a "movie". But, it had warmth, cheer, some good songs, some clever humor, and I liked seeing the character development of a reformed Sunset Shimmer.