Twilight Sparkle is now aware of Princess Celestia's nickname being 'Sunbutt,' and Princess Luna's nickname being 'Moonbutt.'
But the answer to one question still eludes her: Does Cadence have a nickname?
With this in mind, she decides that maybe it's time she paid her dear sister-in-law a visit...
(Takes place before Season 6. Reading the prequels is not required.)