It's been one year since Rin moved to Tokyo for high school. He was told by the landlord of his apartment that he didn't have to pay rent if he takes care of the landlord's granddaughter, which he immediately accepted. However, that granddaughter, Mako, ended up being a 20 year old college student who is completely incapable of taking care of herself. Even so, it was love at first sight for Rin, and he's been taking care of her ever since.
