In recent years, Mahiro has withdrawn into his world of video games, manga, and television, putting aside his responsibilities as Mihari’s older brother.
From a very young age, and despite being the eldest son, Mahiro has had trouble keeping up with the triumphant pace of his sister, who stands out in everything she undertakes.
Now, his sister has become a scientist, and one day she decides to use her own brother as a guinea pig for an experiment.
Mihari uses her knowledge to develop a strange potion, and doesn’t hesitate to put a few drops in Mahiro’s favorite drink.
The next day, Mahiro wakes up late in the morning and notices something strange on his body, starting with the size of his hands.
Getting up, the first thing he does is look for one of his electronic devices, but in the reflection he sees the face of a girl.
The surprise provokes Mahiro to check what is under her underwear, at that moment Mihari enters the room, and does not hesitate to tell him that he is now part of one of her experiments, and that she only needs to monitor and evaluate his behavior while she researches for a cure, he, or she, will have to learn to behave.
At first, Mahiro doesn’t understand why his sister used him, but when she tells him that she will cure him, he doesn’t care and asks her sister to leave the room to start playing.
The next morning, when checking it out his body, Mihari discovers that her brother has not bathed in days and immediately forces him to take a shower, at that moment, Mahiro begins to discover his new body, and remembers what he has lost.
Mihari seizes the moment to force him to wear underwear and a dress, and in the process, orders him to start changing his bad habits, get up earlier and start his day by washing himself properly.
Days later, Mahiro begins to feel different, and even gets frustrated with his adult games. Mihari tells him that maybe it is natural, after all, he’s a girl now.
To change his mood, Mihari decides that it is time for her brother to do something different and after two years of confinement, she invites him to jog.
And as he remembers how athletic his sister is, he agrees to his sister’s suggestion, albeit grudgingly. On reflection, Mahiro thinks that maybe he has found a role in which he can do something positive, and that maybe it’s time to stop pretending to be the big brother and accept himself as he is right now.
Along the way, he discovers that physical activity without proper underwear can be an embarrassing and painful experience, so Mihari takes him shopping.