Twilight wants to reestablish the close relationship she had when she was Princess Celestia’s personal student. But, shared trials will require them to become much more than faithful student and immortal mentor.
Twilight panics when she discovers that Princesses are expected to have formal dates for Hearts and Hooves Day, so she travels to Celestia for advice, getting a lot more than either of them bargained for in the process.
Celestia and Twilight are in a relationship. Twilight wants to meet Celestia's parents. Celestia, understandably, is terrified. After all, it's always awkward when one's parents aren't exactly ponies at all.
Celestia has seen a lot of terrifying things, but nothing she has ever faced is more terrifying to her than this: Telling Twilight's parents she's in love with their daughter.
Lovers have their reasons. To Twilight Sparkle, the reasons she loves Celestia are perfectly obvious. And to Princess Celestia the reasons why she can't love Twilight back are equally obvious.
It has been over 100 years since Celestia and Twilight's relationship collapsed, since they've seen or spoken to each other. But when Twilight appears in the royal palace, that century of silence will come to an end. Very, very politely.