Murakami and Nakabayashi are just policemen trying to do their jobs. When there is a bomb explosion inside Shuei Primary School, they are asked to do their jobs as policemen and check out the lead. It turns out a doctor from a mental institution called in to warn about the explosion immediately before the bomb went off. And so begins their association with the nameless mental patient whom they would eventually call "Maki."
For some reason, Maki can "speak" with spirits. There are spirits all around us, and Maki can communicate with them. These spirits tell Maki about deep dark things... things that go on when no one else is looking...

(From Selene Scans)