Mizuki Miyama is a picture-perfect student and school idol at Seihou Girls Academy. As student council president, she makes sure everyone in the student body is kept safe, especially her naive sister, Manaka. Mizuki is shocked though when she learns her sister's after-school activities involve sextra credit with her teacher and his friends! She'll do anything to protect her sister's secret, even if it includes sleeping with the campus sleazebag gym teacher!