Princess Celestia violates Star Swirl's privacy. After asking for his forgiveness, Star Swirl the Bearded reveals to her the true nature of the secret she's uncovered.
Then the wizened mage makes an unusual request: pay his mercy forward to her prodigal students in the distant future.
For those less familiar with the show, the cover image shows Starlight Glimmer and Sunburst, younger mages who are referred to in part of the story.