Twilight loved Rarity.

This was not in itself a secret, except of course to Rarity herself.

This is one of those splendid romance stories where one character is madly in love with another character, and where the latter character—the recipient of love—remains miraculously ignorant of said affection until the final words have struck.

To elaborate, Twilight wanted to tell Rarity about her true feelings, yet she couldn't. She was, however, a remarkably bright mare whose head got regularly flooded with ideas. One of these ideas, which she would ultimately realize, was for her to practice her romantic confession. With whom would she practice, though? Certainly not herself, for she stood still like some poor paralyzed animal when speaking her lines to thin air.

Thus, she needed a partner.

One of her friends who sympathized with her feelings, perhaps?

But no, an actor would do. Specifically she would seek out the finest of all actors: a changeling.

Mirror Shade was one such changeling. She belonged to the community of changelings under Thorax's leadership; she also thought of herself as an impeccable performer. When Twilight consulted Thorax, and sought a changeling who would act as Rarity for the right price, Mirror Shade practically jumped at the opportunity.

So an arrangement was made.

Twice a week—once every three days—Twilight and Mirror Shade would meet at the Castle of Friendship. Where they would go from there depended on both the real Rarity's whereabouts and whatever Twilight had in mind.

As Twilight would say with geeky enthusiasm, "It's a lot like tutoring! Isn't that exciting?"

Mirror Shade went along with it; she found Twilight to be eccentric, but charming, like one of those virginal romance novel protagonists. Scatterbrained, but determined. Beautiful, but sexually inert.

Within a few sessions, or rather a few so-called dates, Mirror Shade got to know Twilight a great deal. She saw how Twilight liked to sip her tea, how she tended to her books as if they were foals borne from her own womb, and how she almost took pride in the waves of crimson that sometimes swept across her fine cheeks.

Naturally, to fulfill her role in the romance novel that had become her life, Mirror Shade took to knowing a lot about Rarity as well. Most of the information came second-hand, but when she could get away with it she observed the real Rarity's actions, mannerisms, demeanor, and so on. An actor, particularly one specializing in mimicry, had to be observant at the end of the day.

At the end of the day, she didn't mind how Twilight treated her either. When Twilight confessed her most flowery and furtive affections to faux-Rarity, the experience—once merely tolerable—soon became pleasurable.

Within a couple weeks Mirror Shade started anticipating her sessions with the bookish mare she had come to know rather intimately—although maybe not as intimately as she would like. At the end of each date she would ask Twilight, with a slightly increasing sense of urgency with each occurrence, "Think you're ready to tell Rarity now?"

And every time Twilight would reply with some variation of, "I could... but m-maybe I shouldn't. The time doesn't seem right, you know?"

The sessions continued.

In all her time with Twilight, Mirror Shade sensed that she was being afflicted with a breed of guilt that she had never encountered before.

While she had partaken in all manners of debauchery, things only to be expected from an actor so skilled in fulfilling other's wishes, she never more strongly wanted to be found out than now; she figured, with some hope, that at some point the real Rarity would discover her pseudo-dates with Twilight, forcing the charade to end, and for all parties involved to be honest with each other.

Yet such a discovery never happened.

Mirror Shade realized, with some dismay, that she was too good at what she was doing.

Whenever she shared disgustingly adorable pet names with Twilight, or enjoyed a mid-afternoon lunch with her, or felt Twilight's cute little snout brush against her cheek, she felt as if a dishonorable affair were taking place. It was all so horrifically thrilling.

Then, after all this time, something awful stirred within Mirror Shade's heart.

Something cracked, or flipped like a light switch.

During one of their sessions, in the coda to the day's date, Mirror Shade disintegrated her Rarity facade and cried—as if forgetting that she had become herself, "Oh darling, how much I simply love you is too much for me to bear!"