お兄ちゃんはおしまい！

Mahiro Oyama is a shut-in otaku who has barely left his room in years, while his younger sister Mihari is a prodigy working as a researcher. One morning he wakes up as a young girl, the result of one of Mihari's experimental drugs and her plan to pull him back into the world. Mahiro now has to learn a new body, enroll in middle school, and put up with Mihari's doting as the sister he never expected to be.