Mamoru's mother died shortly after giving birth to him. Now in high school, Mamoru has a crush on Ami, a girl who looks just like his mother did. But one day, Ami asks him a shocking question: "Would you rather if I was your sister, or your mother?" It turns out Mamoru's dad was attracted to Ami for the same reason Mamoru was! They've been dating for a while, and Ami was seriously thinking about marriage. But it also turns out that Ami's mother looks just like Mamoru's mom, and Mamoru's dad was dating her, too! All of a sudden, things are getting very complicated for Mamoru.