Twilight Sparkle, the prodigal princess, has finally returned home from her extraplanar Sabbatical. At her side is a new apprentice, one newly minted Passport from a world very different from Twilight's own. Where once the Princess of Friendship sought to leave Equestria behind forever, now she seeks to return and see those she loved and left behind.

But the journey from the edges of the World That Is to Canterlot is perilous, full of horrors and dangers alien both to Equestria and Passport's own world. But with luck, and her teacher and employer's power, Passport might just survive the experience of being a pony long enough to meet the one pony on Equestria more important to her than any other.

Celestia.