本是同根生，相煎何太急？但如果相同的只有半條根，情況能否改寫？本應少女情懷總是詩的初中女生美奈子，阿媽一早離她而去再覓第二春，與再婚的爸爸同住，得到的除了父愛，還有一個「哎吔」細妹阿優。一次暑假，一間海邊小屋，讓陷入成長困惑的美奈子和夢想成為芭蕾舞舞蹈員的阿優，重新認識和了解自己的半個姊妹，更令美奈子重拾遺失已久的笑臉，體會細緻動人的姊妹情深。導演首作已經綻放神采，鏡頭雖是冷靜，但感情卻一觸即發，甚有大師風範。

Minako is at the age where her father is more like an alien to her than a person. The situation is exacerbated when he comes home drunk one night and tries to climb on top of her. Minako's mother deserted them for another man when Minako was a child. Now working as a hostess, she has little time for her own six year-old daughter, Yu, Minako's half sister. Both girls are yearning for their mother's love. Minako runs into Yu at a game center in the mall and proposes they take a trip. She changes out of her school uniform and into adult clothes and the pair hops on an overnight bus out of the city.

父親と2人で暮らす中学生の美奈子は、幼いころに母親に捨てられたというトラウマを抱えながら生きていた。ある日、幼なじみから、美奈子の母親を町で見かけたと知らされ、母への憎悪と異父妹の優に対する嫉妬を抱く。そんなとき偶然、町で優を見かけた美奈子は、優を誘って亡き祖父母が住んでいた家に向かう。