This is a prequel to the Ga-rei manga (which is a really brilliant series by the way and well worth reading if you liked this show). The show centres around two young girls in a very powerful family of exorcists as they work for a secret government agency to keep the world safe. The shoe basicly tells of how Yomi and Kagura met and then how Yomi slowly descends from a warm loving exorcist on the side of good into an evil supernatural creature that Kagura is forced to kill.
This is very clever because it gives you a level of background information to the manga if you have read it but is lso a great stand alone show if you haven't. The characters are very good as is their dvelopment and this is made infinitely more watchable by the very clever script. All in all this is an excellent show with something for everyone.