As summer vacation begins under one shared roof, Kouki settles into daily life with the four sisters whose parents have entrusted him with their care. The eldest directs him through chores from her relaxed perch, while the second-born offers constant pampering and the third maintains the household with quiet efficiency. The youngest treats every moment as an opportunity for playful mischief, and their individual attentions soon hint at deeper intentions that leave Kouki weighing the future ahead.