When the Mane 6 get swapped out with Mario on his quest to save Princess Peach, and chaos erupts everywhere. Equestria must now be saved by the hero of the Mushroom Kingdom, and vice versa.
When Celestia finds a old rusty spaceship that crashed on Equestria, she finds a star foal called Luna. She goes with Luna to try and find her mother, and it will be hard for both of them.