A reckless mother with 5 daughters lived next door to a reckless father with 4 sons. The daughters wished for a boy in the family, while the sons wanted a girl. So their parents proposed that they swap one child between the two families for 3 months. Kyouichi, the second-oldest son, was selected and the girls, who were only osananajimis before, now treated him like their own otouto or onii-chan. The girl who was swapped out also had feelings for him and was extremely jealous that she cannot be part of the family as well. He felt both happy and very uneasy... how will this pretend-family turn out?