Disaster strikes Equestria twice: the realm is invaded by mysterious black ponies, whose goals are unclear and origin unknown. At the same time, the Elements of Harmony have disappeared, to places far beyond the reach of even Celestia and Luna. As the nation descends into war, Celestia, Luna, Twilight and the others must fight to prevent their home from being conquered by the enigmatic foes.

But it is worse for Pinkie Pie: haunted by dreams of the black ponies, she must venture alone to seek out and retrieve the Elements, for only she possesses the mysterious power that can save them all: the magic of Paries Quartus, the ability to transcend the boundaries of space time...that which pony philosophers and poets have referred to as "the Fourth Wall."