Akihisa opens the door one day to find that his older sister Akira (with giant E-cup breasts, as the narrator points out) has returned home from America to score Akihisa's progress living at home alone, and determine if he should be trusted on his own. For some reason, she appears in a bathrobe, and threatens to kiss Akihisa's if he doesn't remember the Two Rules she gave him before leaving.
1: Only play a maximum of 30 minutes of video games each day.
2: No "deviant" behavior with any women (but men are acceptable).
After she deducts "points" from some arbitrary total for violating these rules, Akihisa tries to stay away from his own house, but the text he sends to Yuuji is misinterpreted in a way that makes Akihisa appear to be having an affair with him, angering Mizuki, Minami and Kubo (who pops up to voice his displeasure). Therefore, Akihisa's classmates decide to visit his house for a study group session, and to confirm whether or not he is having an affair with someone. After a few more misunderstandings, Akira shows up with several bags of groceries.
Akihisa and Yuuji try to cook for everyone and study for the upcoming test, but Akira seems committed to embarrassing him, including the discovery of porno mags under his bed (that do not include brother-sister incest, to her dismay), and the allegation that he couldn't possibly have any girlfriends (for which she deducts more points from her running total). Eventually, Akihisa decides to study hard for the upcoming test in order to score high enough that his sister will leave.
The day of the test arrives and Akihisa seems to be doing pretty well on his answers, but his stupidity hits him again when he realizes after the test is over that he entered his first answer in the space for his name, pushing all of his other answers up one space and getting all of them wrong as a result. Akihisa tells his sister that he tried his hardest, but she responds that effort does not matter, only the results at the end. Akihisa finds out what she meant when he opens the fridge and discovers Akira's failed attempts at making paella, the dinner he had made for everyone at the study group before.