When Princess Celestia comes down with a mysterious illness, Sunset takes it upon herself and Twilight to get to the bottom of it, which would be hard enough if she wasn't dating the other Twilight in secret.
After developing a new portal that can explore other dimensions, Twilight and Sunset are pulled into an adventure, where they meet even more doppelgängers of themselves. Together they have to save the day, and learn about each other in the process.
As graduation draws near, magic begins to awaken in the human world, drawing out old rivals and opening the doors to new adventures. It's up to Sunset and her friends to keep Canterlot safe, all while dealing with their looming futures.