After being shown Friendship by Twilight and her friends, Luster Dawn had quit the School of Magic and moved to Ponyville to stay with her friends. However, Twilight still wants to hear about her best student's lessons from time to time.
Princess Twilight Sparkle is hungry at 1am, and she’s in the mood for hayburgers, so she has some sent to the royal castle. And that’s it. Nothing else happens.
At Discord's urging, Princess Twilight hops on through the mirror portal to Canterlot High, which had been closed for two millenia. Instead of a school, she finds herself in a cargo bay of a starship. Discord also has popcorn at the ready.
The ES Harmony, the pride of Princess Twilight Sparkle and Equestria's first FTL capable starship. It is due to make its maiden voyage to travel further than anypony before it, and yet its first mission... may be its last.