Celestia tasks the mane six, specifically Fluttershy, with reforming Discord, so that his considerable magic powers can be used for good. The incarnation of Chaos has no plans to play by anyone else's tune, but slowly Fluttershy's sincere friendship gets to him. Discord comes to realize that if he keeps doing whatever he wants he will lose the only friend he's ever made. By the end of the episode he has decided to use his magic for good, because having a friend is more important to him than even his own unbridled chaos.
This is a difficult one to review. I know a lot of fans were excited for the return of Discord, after all, he made a strong impression in the season two premier. As for myself, while I enjoyed the character, I felt that his part in the story was done, and that it would be best to leave him where he had been left. So I wasn't nearly as excited as about this particular episode.
However, give that was probably not an option, what we do get is a pretty well put together story. While Discord's turn to good may seem a little sudden, it feels believable enough. Granted I would have liked a few more hints, and there were a few, that Fluttershy's friendship was having an effect on Discord. That would have made the ending feel a lot stronger.
Fluttershy was great in this episode. Were I one of the writers I could not think of a character more suited for the role she played in this episode. Where Spike at Your Service molded Spike to fit the role he needed to for the story, Keep Calm and Flutter On takes the right character for the right job. This is how you tell a story with a multi protagonist cast. You start with your story idea, and then pick which character suits which role best. Not the other way around.
The rest of the mane six were a bit off this episode. While their behavior made sense considering their history with Discord, I did get the feeling that they weren't even trying to reform him. They spent the entire episode trying to point out that Discord was in fact doing everything they were expecting him to do. Though while this does give us the great scene where Fluttershy calls them out on not even trying, it does mean that the rest of the cast take on a much more antagonistic role than normal.
Discord is not the true antagonist for this episode, he's the goal. Fluttershy is the protagonist, with her goal being to reform Discord, while the other ponies at times seem to be working counter to that goal, preferring to just say they tried and seal Discord back in stone. And while they don't become mustache twirling villains, or even villains at all, they do take on an antagonistic role in regards to the protagonist's goals. And that is such an odd concept that I'm not sure if it's genius or not.
Ultimately I am left torn by this episode. I quite enjoyed it, I always like to see Fluttershy get her own episode, but at the same time I feel that the episode bit off more than it could chew. However, I can't penalize the story for being too ambitious, unless it fails spectacularly. Keep Calm and Flutter On does not fail spectacularly, it doesn't even fail at all. But the implications of what we've seen in this episode are so large that I can't even process them. Does this mean Discord will be a common feature of the rest of the series? Or will he make small appearances like Luna? If he's free and reformed, what exactly should we be expecting for this finale?