Mario and co. are ready for a relaxing vacation with Peach's royal friends in Equestria. Little do they know that something so innocent will inevitable lead to their next big adventure.
A year after the battle of the bands, graduation nears, a familiar face returns, a pure heart is corrupted, enemies become allies and the fate of all reality hangs in the balance.
Sunset Shimmer finally returns home to Equestria to right her old wrongs, unfortunately the sins of the Human World follow her back and put everypony in Equestria in danger.