The plot: The bulk of the story is contained in the first chapter, which begins with Twilight Sparkle waking up from an erotic dream involving Luna. At first, she merely finds herself overwhelmed by the power of her irresistible new fantasy, constantly having to masturbate to the detriment of whatever task she is trying to accomplish at the time; but when the next night draws closer, she concludes that she must not allow herself to rest until she's successfully rid herself of her fantasy, lest she dream about it and attract the attention of the princess of the night, to her utter embarassment. In order to get the fantasy out of her system, she decides to write it down.
Meanwhile, her increasingly erratic behavior not only arouses her friends' suspicion, it also keeps Spike awake at night, and when Twilight refuses to talk about what troubles her to either him or her friends, he eventually writes to Luna to ask her to investigate. And so when Twilight finally succumbs to sleep, the inevitable happens: Luna learns of Twilight's fantasy, and in a twist of irony, she does so precisely because Twilight worried so much about it beforehand.
Fortunately for Twilight, Luna – princess of the night, and thus presumably night-time activities as well – quickly warms up to her advances. Unfortunately for Spike, Twilight still keeps on keeping him awake as a result, moaning and mewling in her sleep every night.
In the short second chapter, a frustrated and sleep-deprived Spike writes to Celestia, who has a conversation with Luna and quickly works out the truth; but all the complications that could unfold now are eschewed in favor of ending the story on a silly note, with Celestia, despite initially being angry after discovering the truth, involuntarily finds herself turned on by the thought of going to bed with Twilight herself, in much the same uncontrollable manner that hounded Twilight throughout the story.
My thoughts: The story was well-written and kept me entertained, and the idea of Twilight writing erotic fiction involving Princess Luna was quite amusing. Of course, a good explanation for why she would do so is crucial, and I felt the lack thereof was one of the story's weak spots. Twilight's initial night-time fantasy didn't need to be elaborated on; such things happen. But it was far less clear why she was continually aroused to the point of being unable to focus on anything else. One could come up with any number of explanations for this, but the story provided none, and given that this was both the major plot device and an frequent source of comedy, I felt that the lack of an explanation weakened the story's foundation.
Twilight's interactions with her friends could also have been fleshed out more. Twilight's attempted baking session with Applejack ends as soon as Twilight has to retreat to masturbate, and Applejack, despite being highly suspicious, does not see fit to check on Twilight either then or later on. When Twilight happens to overhear the rest of the Mane Six talking about what clearly is her, nobody shows any initiative to act; the sole exception is Pinkie Pie, who confronts Twilight in her usual inimitable, direct manner, but even Pinkie simply disappears when Twilight runs away, and does not make another appearance in the story.
The only pony who appears to care enough to try and actually help Twilight is Rarity, who comes in in the evening with a mug of steaming hot java and a determination to find out what's ailing Twilight. But her efforts are deflected when Twilight blackmails Spike into shooing her away, which he promptly does and which she allows to happen after he comes up with a transparent lie.
Despite this, the story was entertaining, and the fragments of Twilight's story that obviously lampooned actual erotic fanfiction amused me to no end. I also liked how Luna, upon finding out the secret Twilight fought so hard to keep, was not upset — and was, in fact, happy to play along after getting over her initial surprise. I've always pictured Luna as a complex and multi-faceted character: cooler and more distant than the warm and open Celestia on the outside, much more aloof and inapproachable, but also very sensitive deep down, troubled by guilt, uncertainty and anxiety and occasionally very heavy-hearted, and certainly also with a strongly visceral hunger for life. Of the two princesses, I think Luna would be more willing to share her bed with another pony just for fun.
But while writing a comedy story is easy, writing a good comedy story is hard. Being riotously funny is a fine art, and the story often settled for silly instead; not something I'd fault an amateur author with, but noteworthy nonetheless. On the flipside, I also sometimes felt that maybe there is a more serious story buried in this one, one with less silliness and comedy and a stronger emphasis on not just the relationships, but also the complications they'd inevitably bring. But that would've been a very different story, and I cannot hold it against the author that they did not set out to write that story instead.
Overall verdict? If you like comedy romance, check it out. There's nothing explicit that goes beyond a tender kiss, either, so don't let that keep you away.