Hitotsu Yane no Shita no

High School freshman Yuzuriha Kyuuta made a promise with his childhood friend Ririko to marry her when they grow up. Being separated from her since elementary school he now enrolls at her school, a former all girls private school for rich young ladies which has just opened its gates for male students, to meet her again. But upon meeting her, it turns out he has to live in the girls dorm and she is the dorm head!? And what's more, she doesn't seem to remember him at all! Just how will Kyuuta be able to keep his promise?