Sakai Yuji is a normal kid with normal ambitions and expectations from his life. One fine day, though, en route to school, everything around him freezes and he appears to be the only one capable of motion as a giant doll-like creature starts devouring the helpless humans. As it approaches Yuji, a mysterious girl clad in black, with flaming red hair and a literally flaming sword cuts it down. The girl, Shana, explains that she is a Flame Haze, a hunter whose job is to track down and kill the Denizens of the Crimson Realm (i.e. the frozen world, which is, you guessed it, tinted red). Also, turns out that Sakai Yuji is actually dead, and this Yuji is just a temporary copy of the original.
The anime starts off brilliantly, introducing a highly interesting villain and spinning a horror-story with marked success. A third of the way through, though, it starts losing steam, and the genre veers from horror into a strange mix between action and slice-of-life. The plot wouldn't survive close scrutiny, so it's advisable to avoid looking for holes - there are numerous. It's not really as though the characters are very unique either, which leaves a major question: why does this show have three seasons?
To be honest, yours truly isn't quite sure. One theory is that this anime has at least one quality appealing to almost everyone. There are villains with evil goals, a love triangle, conflicting views on the value of a Torch (a 'replacement copy' like Yuji), rivalry between a pair of strong characters, and everyday life anyone can relate to. While Shana herself is typical to the point where she could be replaced with Ayano from Kaze no Stigma and no one would notice, Yuji doesn't take completely to the backseat. He's not a punching bag, and he deals pretty well with the sudden turn his life has taken (a little too well, maybe, depending on your taste). Most importantly, both their powers and skills grow throughout the series, but they never outshine the villains' abilities. The series also does an excellent job of balancing its unusual combination of genres, and is packed neatly into its 24 episodes set.
The soundtracks are nothing you'd want to go back to, unfortunately, but they're not off-putting either, which is more than what you can ask for sometimes. The style of artwork for the characters is the definition of moe (cutesy), but the anime gets serious props for gorgeous backgrounds.
The series doesn't take itself too seriously, so you shouldn't either. It's adapted from light novels, and as long as you don't lift your expectations sky-high, it's relaxing to watch when you have a bit of free time between homework and assignments. If you do enjoy the first season, be sure to work through season two, because that's where the real story - what decent parts there is - lies.