So, at first you think this is a standard 'It's a Wonderful Life'-style plot. It's Christmas and Haruhi is being her domineering self, putting Mikuru in a sexy Santa dress and telling everyone they are having a secret party on Christmas Eve. Kyon goes to bed on 17 December, completely normal... and wakes up on the 18th to realize that suddenly Haruhi and Itsuki have never attended North High, both Yuki and Mikuru are normal high schoolers, as is Ryoko Asahina, who incidentally is sitting in Haruhi's desk.
Though it's not really that plot -- the second half features 'setting right what was wrong' based on whatever clues the original Yuki could leave in the club room and heading back to a time where he can find allies, before the timeline diverges. Kyon hardly questions his realization that he is happier in the original world, and even the idea of hanging with the now-human SOS Brigade members on weekends isn't appealing. Kyon never made the wish that he'd never met Haruhi here, and it turns out the reason he's immune is never really explained -- while the SOS Brigade members all needed a past retcon to be normal, and Haruhi needed to be separated from Kyon, even Kyon's classmates didn't remember things. (Nor is how the instigator got the power to re-write the last year.)
The ending was a bit of a twist for me, at least, and it was a good read, though.