Based on the game by Haikuo-Soft.

A story about the reunion of 4 childhood friends, Yuzuki, Matsuri, Nono and Makoto, in their deserted old school campus, Mochitsuki Academy. 3 years ago, they buried a time capsule under a tree inside the school and made a promise. A promise to meet again after exactly 3 years from that day. And the day of promise has finally arrived.

(Source: AniDB)