The protagonist Nanase Suzuna is a very normal college student. Her father died when she was a child, but suddenly one day her mother introduced her to the person she was going to get remarried to. Because of her mother's happy smile and how nice her new father seemed Suzuna approved of the marriage and she prepared to move into her new fathers house but... On the day of the move, at the imposing gated mansion, she learned that her new father had four sons! Suzuna was shocked! More than that, her mother and new father were going away for a week and leaving her to live alone with her four new brothers (and their servant Kageyama) under the same roof. Then Suzuna received a proposal from those brothers. "If you can make us approve of you as our little sister within one week, we'll bless those two's marriage." She hoped for her parent's happiness so Suzuna happily accepted but... In truth the brothers already objected to the marriage, little did Suzuna know but to them this was just the start of another game. "How about we bet on which one of us brothers Suzuna will fall for first?" It's one week until her father and mother come back, will Suzuna be able to protect herself!? Will she be able to build sibling bonds with her new brothers!? What is the true nature of Kageyama who is always watching over the five of them!? Now this skillfully dangerous battle of one little sister VS four big brothers + one man will start――!