When a portal to another world appears outside Canterlot, the ponies' initial reaction is of enthusiasm, hoping to greet these strange aliens with open hooves. Too bad this world was already visited by another Equestria...
The Elements of Harmony find themselves transported to a world full of evil and darkness. On the journey home they make new friends, as well as new enemies in the form of evil counterparts to themselves.
Humanity's space exploration ultimately took the form of billions of identical probes, capable of building anything (including astronauts themselves) upon arrival at their destinations. One lands in Equestria. Things go downhill from there.