Set between "The Ending of the End" and the flashback in "The Last Problem" The Main 6 are called before the princesses to discover the mystery of the world's first friendship, by solving the world's first friendship problem. In the distant past, the ancient ponies were defended from the dangers of the untamed world by the humans: the mightiest warriors the world ever saw and users of powerful elemental magics. However, something happened between them that drove the humans underground to the very heart of the world.

Down in the cavern our little ponies find a vast but treacherous city, and must work to repair the bonds broken by time through facing dangers untold. However, several unsavory parties seem to want them for their own agendas.