Akitoshi Arikawa is a university student living independently away from parents. His step big sister Midori one day requests him to take care of his step little sister Ichigo's study, because Midori is anxious about Ichigo's bad grades. He is happy to accept Midori's request thinking it'd be good chance to catch up with Ichigo, but...

Ichigo's academic abilities are far lower than his expectation. She shows no interest in studying but cannot get her eyes off Akitoshi's porn books.

"I don't wanna study. Please teach me sex ed instead, oniichan."

Placed before Ichigo's lustful approach, the very last remaining fragment of reason, that kept him in mind even though they are not blood-releated, eventually collapses.

While they are in a stuck because Akitoshi can't overcome his sexual drive, he comes up with an idea to have sex as a reward for successful progression in her study. With this idea, Ichigo's study for entrance exams finally begins.