Nightmare Moon is back from her thousand year exile, and Twilight Sparkle is utterly unprepared to triumph with magic or with words. The two join forces (not that Twilight has much of a choice) to ‘save’ Equestria from eternal night.
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.