This story is a sequel to My Little Pony, Friendship is Magic: Heart of Friendship

A collection of mostly episodic stories taking place after the completion of the Mane 6's latest mission and the humans' great migration from the Heart of the World. Both peoples can have very different values and outlooks, but life would be no fun at all if everypony, everycreature, and everyone were exactly the same, right? Old enemies might become new friends as new enemies make trouble that only multi-species teamwork can thwart.

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.