In the halls of their elite academy, the Nanase sisters stand out as the school's most admired idols, drawing admiration from every student including the reserved Sousuke. His quiet crush on Arisa remains unspoken until Mika surprises him with an after-school meeting that upends everything he knows. Their parents have married following a chance encounter abroad, instantly turning the three into siblings who must now share one household. As they adjust to this sudden closeness, Sousuke faces the everyday challenges of his transformed family life.
