Shota is left in the care of the neighboring Kominami family for three days while his mother goes overseas. Next door lives Hikari, a kind and lovely older sister figure, whom Shota mischievously teases—slyly touching her big breasts and playing harmless pranks. He runs wild throughout the house, sneaking into the kitchen, bathroom, living room, and bedroom, acting on his childish impulses. Hikari forgives his antics, treating them as innocent child's play. But when Shota gets carried away as usual and pulls out a condom asking, "How do you use this?", things take a daring turn.