Twilight's caught in a Groundhog Day timeloop, but with a harsh twist - just five hours or so after the beginning of the loop the Changelings brutally invade Equestria again, and Twilight generally dies right afterward. This gives the story a much tighter and clearer plot than Groundhog Day. There's a definite puzzle that needs to be solved; the Changeling invasion must be thwarted, and Twilight has only five hours or so in which to do it.
As might be expected from that premise there's rather a lot of violence in this story. It's handled well, though, and the psychological damage of dying over and over and over again take their toll on Twilight without becoming overwhelming. It's quite interesting seeing all the different strategies that Twilight tries before finally finding the "perfect run".
An interesting feature of this story is that it's written in the first person with Twilight as the viewpoint character. As such, it really gets "inside her head" and in my opinion does a great job of it. It does go a bit outside the bounds of the character as defined by the show, but given that she's put in circumstances well beyond what the show puts Twilight in it seemed fair.