Kyousuke Kousaka arrives from school and notices his 14-year-old sister Kirino chatting on the phone. Upon seeing her, he thinks of how refined she is, judging from the dyed hair, accessories and the make-up she wears. However, he also points out that he is not intent on boasting about having such a beautiful sister. In fact, he understands how other boys would be jealous at having a sister like her, but in reality, he is not comfortable of her presence.
After being ignored over someone on the phone, Kyousuke goes for a short drink and hears his sister planning on going out... at night. Kyousuke doesn't want to think too much about it and goes upstairs.
Kyousuke calls himself "an average high school boy". He does not prefer to be in any special club or to call his usual listening to music and reading manga to be some hobby of his. Aside from school, he sometimes hangs out with his friends and, at home, watching TV or read manga. He admits that his lifestyle can be boring, but he takes importance in being normal, as it would be the safest route.
A change of clothes later, he plans on going to the toilet one last time before studying. On his way down, he collides with Kirino. It wasn't enough for either of them to lose balance, but it caused Kirino's bag to fall and spill its contents, which among them is some cosmetics. Kyousuke offered to help, but had his hand swatted away by his sister, who tells him to not touch anything.
Kirino heads off for the front door, and Kyousuke couldn't help but feel annoyed at the actions of his sister, which are as cold as ever. However, he has coped with her tendency to be unfriendly towards him and just continues with what he needed to do -- to visit the toilet.
He eventually loses the interest to study after a few minutes of reading manga. Kyousuke picks himself up and, heading into the hallway, finds a white case peeking out from the shoe closet. Curious, he picks it up and is shocked to find out that it was a DVD case, whose case has a drawing of young girl in "suggestive clothing". The title says "StarDust Witch Meruru Limited First Edition".
It was a DVD of an anime series.
As he scrutinizes the DVD, the front door suddenly opens. His mom has just arrived, and he manages to hide the DVD from his mother's sight. Kyousuke tries to keep himself from being suspicious, but he would have a hard time to defend himself, as his mother knows that he has a stash of mature books on his room (his mother usually goes in and cleans his room without him knowing). However, what's important is that he manages to escape without the DVD case being noticed.
Inside his room, he gets his chance to look closely at the DVD case. He wonders who it might belong to; he knows that he doesn't own anything like this, and he can't connect the childish DVD on his parents (and definitely not his anime-hating father) or his sister. His curiosity grows, and upon opening the DVD, he finds out a surprise: that the contents do not match the case itself.
What's more shocking is that the DVD stored inside is entitled ‘Let’s Make Love With Your Sister!”, together with an R-18 rating.
The situation becomes more drastic for Kyousuke, and a desperate scream startled his mother from the other side of the door. He manages to say an alibi before returning to the DVD on his hands.
Determined that the DVD is not his, he decides to look for the owner.
7 o'clock is dinnertime for the Kousaka household.
To his surprise, his sister still follows her 6:30 curfew, and his father has just arrived as well. As their parents proceed to the dining table, he finds Kirino standing still on the entrance for some time.
As dinner starts, Kyousuke notices that everything is like his preferred usual: his father dressed in traditional clothes, his mother looking like a housewife, and his sister eating quietly on her own. Soon, he opens the topic about going to the convenience store and later, the DVD he discovered, leaving out many details of the latter behind. His mother and father talk to him about being careful about anime, while Kirino's chopsticks begin to tremble in her hand. Kirino then storms back to her room.
Kyousuke feels that somehow Kirino responded to his hints, and he is now suspecting her to be the owner of the DVD he found earlier. However, even he has no faith in his own judgment. Wanting to know more, he leaves the table and heads upstairs. Kyousuke then puts his plans into action, taking a detour to the back of the house.
He manages to go back to his room and there, he meets with Kirino, whose face is blue with fright as he searches in his porn collection.