Filla, a demon-in-training, meets the angel-in-training Micael at school and is immediately fascinated by him. When they grow up, they become partners who work together to guide the souls of the dead to Heaven. Filla chases after Micael, without much success. Micael has some problems. He has a lot of guilt because of things that happened in his past that weren't really his fault, and because of that he doesn't want to get close to anyone, to risk hurting someone and being hurt. On top of that, they have the usual problems of angel and demon manga protagonists, with humans in need of help and worlds in need of saving. There is a lot more to the story than that, but I don't want to spoil it for anyone who doesn't want to be spoiled.

I think "Matendou Sonata" is a fantastic manga. It's funny, sweet, and dramatic, and the ending nearly brought me to tears. It suffers a little from neon-light foreshadowing, amateurish artwork at the beginning, and the parts where the author tries to tackle Big Important Issues, where she comes across (to me, at least) as somewhat shallow and simplistic. But it was a manga written for middle and high school students, after all. The characters are loveable, and it is still one of my favorite manga series.

Now, if you want to be spoiled (or you've already read the translations) you can start reading a detailed plot synopsis which so far covers up to volume 8.