Reunited with a childhood friend after five years, I was in my first year of high school, yet you stood before me, a teacher.... Kato is a young man with a peculiar condition that causes him to age three years for every year of real time. Orihara watches over him as both a friend and a student. This is the tale of the unusual life of these two former classmates.
(Source: MangaPlaza)
— Description from AniList community (anilist.co).
