Yuu's entire world collapses in one conversation, and the only solid ground he finds is a girl who turns out to be his stepsister. The movie doesn't wait to complicate things, it knows that's the only reason you're here. What makes this work is that it never pretends the situation is anything other than what it is: two kids caught in their parents' chaos, finding each other at the exact moment everything becomes impossible.

Serving as a prelude to the Marmalade Boy TV series, this film has Yuu looking back on the day his parents announced they would divorce and each remarry into another married couple. Shaken by the news, Yuu heads out for a walk and spots a girl who wins his heart at first sight. She has no idea he is there, yet he keeps following her, and his feelings only grow stronger. Then he learns who she is: the daughter of the couple his parents are pairing off with. His first love turns into a complication, laying the groundwork for the romantic comedy and drama of the series to come.