叛逆的女高中生AMI（竹内结子饰）发现自己是试管婴儿，父亲是个代号为307的精子捐献者，不满于现在的家庭状况的她带凭借自己靠援助交际所赚来的钱，带着智障的同母异父哥哥TAKUYA（安藤政信饰）离家出走，从此踏上了漫漫的寻父之路。本片由樱井亚美的同名小说改编而来，电影内容与原作有很大不同，但却成功表现出了原著中所传达的“封闭世界中少年的危险的感情”。

High school student Ami was born through a fertilization technique. Her brother is developmentally challenged and is immature for his age. They go on a trip and when they arrive at a clinic decide to stay with the doctor and his wife.