Twilight shows up on Rarity's doorstep in distress, which isn't too unusual. Her admission that she's come to Rarity about romance is, however! Naturally, Rarity is all too eager to tutor her in love's ways. It's only practice, isn't it?
Going on an inter-dimensional date with her marefri—sorry, girlfriend—was not supposed to lead to the existential crisis of a lifetime, but there was Rarity anyway.
Turns out Rainbow Dash and Rarity are a married couple, and have been since before Twilight moved to Ponyville. Twilight never knew this. Hilarity ensues, as well as a few feels.
Friends who play together, stay together. That was the philosophy Rarity had in mind when she suggested she and Pinkie play a game of chess every week. Unfortunately for Rarity, however, she finds out that Pinkie is playing more than just one game.