Deep below the earth of Equestria, beyond the sun and the soil, secrets lie buried. When Twilight and her friends become stranded in an ancient civilization, they soon learn that some things are best left a secret. Ignorant of the world above them, the inhabitants of this decaying world of steel and darkness have been buried for centuries. Twilight finds herself desperately trying to understand them and their militaristic way of life.

As the two worlds collide, it’s unclear if the new ponies will prove to be allies or enemies of Equestria, but neither are aware of a greater awakening evil which threatens both civilizations. As the six friends struggle to survive and understand the underground world, the hope of saving Equestria seems more remote with each passing hour.