Prologue: MC has a younger sister named Anna. Anna is a shy introvert who wants to follow her brother's footsteps and become a talented tennis player. MC recommends her to study the tennis footage on their computer but completely forgets about the hidden Black-gyaru folders which the sister ends up finding.
Main story: Fast forward, MC is promoting the tennis club on campus when a Black-gyaru approaches him, and it turns out to be his sister. Later on MC is in his room and is about to fap to Black-gyaru videos, when Anna visits him. She admits that she changed so she can be his ideal woman. MC is super confused and has conficted feelings as he is obsessed with gyarus, but on the other hand it is his sister. Anna gets pissed off at his indecisiveness and gives a BJ and then they have each other's first time. MC is more confused than ever after the deed, but Anna announces that she will be back every day.
Each day, as the sister is coming to his room, you have time for 3 sexual activities from the "menu" and you learn more about both Anna and MC. Anna teases him every day in a playful manner, but it's very clear she is really into him. She studied MC's Black-Gyaru material well so she puts on a good act, but occassionally she slips and reverts back to her shy persona much to MC's amusement. MC is conflicted about his feelings still, so on the final day Anna spells it out for him: she loves him not as a brother but a man. MC finally reciprocates her feelings and they go on their first date.
Endings: The endings are determined by the number of creampies MC gives to Anna during the main story. If MC regularly creampies Anna you get Ending 1, if you pull out most/all the time then you get ending 2.
Ending 1: Anna gets pregnant and ends up dropping out of school, while MC makes his professional tennis debut. He's regretful that Anna had to stop playing tennis, but Anna is okay with it, saying tennis was just one of the connection they shared and now they have a stronger bond.
Ending 2: In the epilogue scene MC and Anna are having at it, and after the sex Anna playfully mentioned that she deleted all of MC's Black-gyaru fapping material as he no longer needs it, much to MC's dismay.