18+ Dorei Sensei

Masato Irie, age 33, is a teacher with a wife and kid. An ordinary man living peaceful days, he cherishes the small happinesses he enjoys with his loving family. However, one day he has a shocking encounter that changes his life. He meets a beautiful young woman... She turns out to be one of his pupils! Her love increases day by day, and his fate is derailed...!!

(Source: Nihon Bungeisha, translated)