Luna had been writing to what she thought was another pony. They had been talking through magically sent messages, for over a year now. When they finally decide to meet, they find out they might have less in common then they thought. Humans are just bugs to ponies, being just a 1/4 of an inch tall. They soon find out that they still have a lot to talk about.
Though, due to circumstances, they may have to talk in person from here on out.