Released first as an eroge - with a clean ver. later - it has been adapted in to 2 anime series, 5 light novel series, 3 audio-dramas and 2 manga series. Contrary to general belief, it's not a shoujo or 'moé' title, but a seinen.
Yuuichi Aizawa - a second-year high school student used to visit his cousin and aunt in their town on holidays, but with an event, he stopped doing it so 7 years ago. Due to family circumstances, he returns to this town to live with his aunt and cousin - also transfering in to Nayuki's school. With his return he encounters a number of mysterious girls. When he starts to get himself involved with these girls, he starts to see through the shadow cast on each of them.
Within a town engulfed in mysterious happenings, Yuuchi has to face both the many anormality surrounding the girls, and his sealed-off memories related to them.