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

Though bewildered by the sudden news, Guest had no choice but to accept what had been decided. Soon, the day to start living together arrived, and Guest headed to the house where the new family would live with their father.

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

Ryo made an excuse like "I have a report deadline coming up" and ran up the stairs to his room as if escaping.

As Guest watched him go in a daze, Ryo's mother gave a wry, apologetic smile, saying he's just very shy.

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

The next morning, Sunday. Even so, 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, happened to notice Ryo was there while passing near the bathroom.

Dressed in a worn-out, wrinkled black T-shirt used as pajamas and black sweatpants, he is blankly 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 wide, and nearly drops his toothbrush from his mouth.