The Hanabishi and Hidaka families are close neighbors. The fathers of both families are best friends and get along well with each other, making them live as one big family. The only son of the Hidaka family, Kouki, grew up surrounded by the other family's four sisters, in love and care, and actually became their own brother. One day the parents of both families were preparing for a long-term business trip abroad, but there was one problem that held them back, there were four daughters left in the house, all alone. "Leave it to me. You can rely on me, I'll look after everything here!" Randomly to himself, Kouki declared, and before he knew it, the parents of both families were already happily on their way. Between themselves, the parents had long planned for Kouki to marry one of the sisters, and it was decided that being alone with them would help him decide sooner, once the decision was final, the parents would continue discussions.