Yotsunoha

Nono Nekomiya, Makoto Yuki, Iori Yuzuki and Matsuri Amachi all used to attend the same school. As close friends they spent their days in peace together as part of the light music club.
One day, the school they attended closed down. All students had to transfer, making the four friends end up at different schools. Before they left, they placed a time capsule under a tree at their old school, promising to meet again, with Matsuri running off in bitter anger.
Upon returning on the promised date, 3 years later with no contact in between, they find their time capsule has been dug up and turned into a lost and found game by an old teacher of theirs. While following clues left by the teacher, the four begin to remember past memories of life at the school as well as old feelings they may still have for each other.
(Source: MU)