Mami Sakurai, an older sister who lost her parents at a young age, has been working hard since then. After graduating from college, she became an office lady (OL). Now married with a husband, they take care of her younger brother Yasuharu, who lives with them while studying for exams. Mami cherishes Yasuharu dearly, treating him like family despite their blood relation. One day, Mami receives a call from Takemoto, a man claiming that Yasuharu had wronged his gang leader's sister. To settle the score, Takemoto invites both Mami and her husband to meet him...

幼い頃に両親を亡くした櫻井姉弟。姉のまみは努力を続け短大を卒業し、OLとなった。今では夫もおり、受験勉強にいそしむ弟の雅人を居候させて暮らしている。まみは唯一の家族である雅人を、夫と共にとても可愛がっていた。そんなある日、滝本という男から連絡が来る。「アンタの弟がウチの組の姐御をキズモノにした」と。落し前をつける為にと、まみは夫と共に滝本達に呼び出され……。