Talk:My Little Pony Equestria Girls: Rainbow Rocks/@comment-24908786-20140716133017

Since that new information was posted, I've come up with a theory to the start of the movie, more or less...

Twilight is enjoying her new life as Princess of Friendship, and the Crystal Mirror has been transferred to her castle from the Crystal Empire. As Twilight and her friends and the Princesses are having a get together at Twilight's castle, Sunset arrives asking for Twilight's help back on Earth at Canterlot High as new trouble has appeared. Once Sunset explains the situation to everypony, Twilight agrees to return to Earth with her and help the Humane Five with stopping Adagio and restoring harmony to CHS.

That's the best I can think of right now: Sunset returns to Equestria to ask Twilight for help, and she returns to Earth with Sunset to help the Humane Five stop Adagio.