After a run in with the Dazzlings where they reveal exactly how much they knew about her past, Sunset did exactly what anypony back where she came from would do. She sang a song about her feelings, letting it all out. What happened at the end of her song though is what is bothering her. She knew her friends at the time transformed anytime they played music... she didn't realize that she could transform too. The problem? She transformed into a humanized phoenix not a humanized unicorn.
In an attempt to learn why she would transform into that, she runs away back to Equestria to do a little investigation into the matter. With her friends from both sides of the portal trying to find out what's bothering her, and give her a helping hoof, I mean hand, can she find out the truth behind Sunset Shimmer...?