Nono Nekomiya, Makoto Yuki, Iori Yuzuki and Matsuri Amachi were close friends who attended the same school. They spent their peaceful days together as part of the light music club until the fateful day the school they attended closed down, forcing all students to transfer to different schools.

All four of them were going to be separated, but before they left they placed a time capsule under a tree at the school, promising to meet again. Matsuri ran off in bitter anger.

Upon returning to the same spot on the promised date three years later, with no prior contact in between, they found their time capsule dug up and turned into a lost and found game by a teacher who used to teach them.

While following clues left by the teacher, the four begin to remember past memories of life at the school as well as stirring up feelings they may have for each other.

(slightly edited)
Thanks frosephtheshadowman!