Discord defeated the bearers and almost destroyed the Elements of Harmony, being them unable to defeat him. The princesses must reveal a secret that only they remembered, something involving the seventh Element, the one that choose Celestia to be it's only bearer and that, years later, disappeared.
Being its location unknown to everypony, the bearers and princesses trough that they lose the fight… until a unicorn shows them how there are still chances to win the battle.

"I must show to this whole world how not every hope is lost…" — Novi Spei.