"I'm here as the collateral for my brother's debt. I will be in your care from now on." Akari Miyamae suddenly came barging in into Motomu Shiragi's apartment. Giving off the impression of a good girl who cares for her brother, and a beautiful girl who would be gossiped about throughout the school, Akari suddenly offers herself as collateral for her brother's loan. And that's just for 500 yen. "Until my brother pays off his loan, I will gladly be your possession, senpai!" she declared happily, for some reason. Motomu was bewildered by the overly wild development, but Akari pushily cajoles him and steadily goes forward with the preparation to freeload. And of course, this whole "collateral" thing has another motive―it's the start of a single-room cohabitation with a beautiful proactive girl!