The Yazaki and Kinomoto families live next door to each other. The Yazakis are four brothers raised by their father, and the Kinomotos are five sisters raised by their mother. Both families have long wished for the child they never had: the Yazakis want a daughter, and the Kinomotos want a son. One day, the parents come up with a wild idea: "Let's each trade one of our kids and have them live with the other family for three months!” And so, the Yazaki's second son, Kyouichi, and the Kinomoto's second daughter, Sakuya, switch places and begin their new swapped lives. The lively Kinomoto sisters are unsure how to treat Kyouichi at first—calling him "onii-chan" feels strange—but they gradually warm up to him. Kyouichi, for his part, is overwhelmed by life in a house full of girls. But even through the chaos, he begins to feel something he's never felt before: the warmth of a family. The three-month deadline draws near. As Kyouichi, Sakuya, and their families spend time together, they begin to realize just how important they've become to each other. When the time is up... will they go back to their old lives, or choose something new?

The Yazaki and Kinomoto families live next door to each other. The Yazakis are four brothers raised by their father, and the Kinomotos are five sisters raised by their mother. Both families have long wished for the child they never had: the Yazakis want a daughter, and the Kinomotos want a son. One day, the parents come up with a wild idea: "Let's each trade one of our kids and have them live with the other family for three months!” And so, the Yazaki's second son, Kyouichi, and the Kinomoto's second daughter, Sakuya, switch places and begin their new swapped lives. The lively Kinomoto sisters are unsure how to treat Kyouichi at first—calling him "onii-chan" feels strange—but they gradually warm up to him. Kyouichi, for his part, is overwhelmed by life in a house full of girls. But even through the chaos, he begins to feel something he's never felt before: the warmth of a family. The three-month deadline draws near. As Kyouichi, Sakuya, and their families spend time together, they begin to realize just how important they've become to each other. When the time is up... will they go back to their old lives, or choose something new?