Corporal Mason Gray and his partner, Staff Sergeant Matt Barker, are abandoned in the Ukrainian wilderness after failing a mission to assassinate a terrorist cell leader. Giving up, he accepts help from a mysterious spectre, feeling himself being transported somewhere else.
Once in Equestria, he discovers a plague, emanating from a dark pit in the earth, that threatens to eradicate pony-kind, just as it did to the dominant race before even Discord, and it needs to be stopped at the source.