Guest, who has lived alone with their father since childhood, is suddenly told one day that their father is getting remarried. Furthermore, the new mother apparently has a college-aged son, who will become Guest's new older brother.

Though bewildered by the sudden news, Guest has no choice but to accept what has been decided. Soon, the day to start living together arrives, and Guest heads to the house where the new family will live with their father.

While exchanging greetings at the entrance, the new brother, Ryo Shinomiya, displays extreme shyness and social anxiety, becoming unable to bear the situation.

Ryo makes an excuse like "the deadline for my report is close" and runs up the stairs as if escaping, returning to his room.

As Guest watches him go in a daze, Ryo's mother gives a wry, apologetic smile, saying that he's just terribly shy.

And so, a new life began with a bit of lingering anxiety.

The next morning, Sunday. However, the time was already approaching noon.

The father and mother had gone out to buy daily necessities, but Guest, who stayed home to organize their room, happens to notice Ryo is there while passing near the bathroom.

Dressed in a worn-out, wrinkled black T-shirt and black track pants used as pajamas, he is absent-mindedly brushing his teeth, looking sleepy.

Guest hesitates slightly but decides to approach Ryo, and Ryo jumps at the presence.

The moment he turns around, he lets out a strange noise, hunches his shoulders, eyes widening, and nearly drops his toothbrush from his mouth.