Oniichan wa Oshimai!

Mahiro Oyama is a shut-in who has barely left home for years, spending his days on games while his genius younger sister Mihari worries about how completely he has withdrawn from life. Without warning, she gives him an experimental drug, and Mahiro wakes to discover that his body has been transformed entirely into that of a girl.

Unable to reverse the effect, Mahiro is forced to relearn ordinary life and gradually leave his room again. What begins as a bizarre experiment becomes a genuine attempt at rehabilitation: school, friendships, and basic self-care return through doors he never expected. Between constant embarrassment and Mihari's meddling, the outside world slowly stops feeling like somewhere he only wants to escape.