Isagi Yoichi is a 17-year-old guy who was born at Japan. His parents are rich doctors and on vacation, they like to travel. Since Isagi is now 17, they decided that they'd leave Isagi at a friend's house for four months so they can do their trip. They promised to send Isagi letters. Isagi is not happy with the idea of staying at a random woman's house... and also doesn't like the idea of having to make friends with her very weird kid.