自幼失去双亲的江上盟（オダギリジョー 饰）和妹妹江上飒（長澤まさみ 饰）是一对性格截然相反的兄妹，为了让哥哥好好读书，将来当成功的医生，小飒自小便为哥哥付出。但调皮的飒却是生活精明读书不灵的女孩，最大的梦想是在30岁前存够1亿日元，于是她在夜总会打工，借钱给姐妹们收取利息来赚钱。 年纪虽轻但已然成为副教授的盟，生活中是个淳朴的人，当他发现妹妹正与有妇之夫交往时头疼不已。这时他也被卷入一场因偶遇引发的事件中，桐原里子（友坂理惠 饰）与盟发生关系后向他借取300万支付父亲的医药费，护兄心切的飒查出里子所说的一切全是谎言。矛盾的盟到了约定地点戳穿里子的谎话，但最终还是决定回头问个清楚的他却发现桐原里子坠楼身亡。而他成为最大的嫌疑人，而飒似乎亦与桐原里子有过非同一般的接触。 兄妹的感情与信任能维持不变吗？事情的真相到底又是什么呢？Egami Mei is a genius surgeon, but is somehow still lacking. On the other hand, his younger sister Satsuki is poor at studying, but she has a good nature and a strength for living. "Boku no Imouto" is a human drama about these two siblings whose parents died while they were still children. Because of their different values, they are always arguing, such as when the brother finds out that his sister is having an affair with a married man. However, the siblings have also developed a strong bond that keeps them together despite their constant conflicts. But a huge and unexpected incident occurs, which threatens to shake their relationship.
