Akitoshi is a fat, NEET, nerdy older brother who is constantly treated badly by his younger sister, Ayaka.
One night, when both parents are away on a business trip overseas and the two are left alone, Akitoshi sneaks into Ayaka's room for some trivial reason... and ends up cumming on his sleeping sister's face.
He tries his best to cover up the evidence, but the next morning...
"You came into my room last night, didn't you?"

Found out!? Ayaka turns to a panicked Akitoshi, shouting, "You underwear thief!"

Instead of cumming on her face, she brands him as the underwear thief.
It's true that he's secretly borrowed clothes many times in the past, but not yesterday! Akitoshi searches for the laundry basket, hoping to clear his name of the false accusation.
The underwear is easily found, and it turns out that it was a misunderstanding, but
"Why should I apologize!? I doubted you because you looked like a underwear thief!"

Rather than apologizing, Ayaka loses her temper and starts hurling insults at him...

"If you hate me that much, I'll turn you into my meathole before our parents get home!"

The creepy brother's lewd behavior with his sister begins...!