Daiki Rusuhara, a NEET candidate who has failed the university entrance exam twice and is now a ronin, living at home. His father has passed away, and he lives with his mother and younger sister, Mao. While not exactly harmonious, they have been living a peaceful life.
However, Mao, who once innocently admired him, has become a "mesugaki" (a derogatory term for a spoiled and disrespectful girl), constantly provoking and mocking him. Furthermore, her chest has grown significantly in recent years, causing him to feel agitated not only in his heart but also in his "chin-chin" (slang for penis).
"Don't mess with me... with your disrespectful attitude and big boobs..." he thinks to himself repeatedly.
Perhaps because of this, one night he dreams of Mao, transformed into a devilish figure with horns, a tail, and bat wings, and he is helplessly forced to ejaculate by her.
Unaware of this, Mao visits his room, mocking him for filling the trash can with used tissues and laughing at him for having an erection in the morning.
"Don't you dare, you arrogant little brat with your huge boobs..."
I was constantly irritated by my little sister's attitude and her large breasts. One day, my mother told me about Mao. She said that Mao had been sleeping a lot in class for the past month and had been going out after school without telling anyone where she was going. My mother asked me to keep an eye on Mao and then left for a business trip. Even though she was arrogant, she was still my sister, and I decided to take care of her while my mother was away. I was grateful for my mother's kindness and encouragement, even though I felt a little ashamed of myself for being a second-year college dropout.
I tried to gently tell Mao to stop sleeping so much, even at home, but she retorted, "I don't want to hear it from a loser like you."
But it wasn't just the sleeping. Mao was napping on the sofa, and her body was gradually becoming transparent! What the hell was going on?
Startled, I shook Mao awake. She looked at me with concern, but then quickly went back to her usual self, teasing me, "What are you daydreaming about?"
I was annoyed and went back to my room. But when it was time for her to demand dinner, she wasn't around. I started to feel uneasy and went out to look for her. She wasn't anywhere in the house.
I couldn't shake the image of my sister's body becoming transparent right before my eyes. It wasn't a hallucination. I ran through the streets, desperately searching for Mao, but she was nowhere to be found.
You return home, plagued by anxiety and utterly exhausted, and soon fall asleep as if you were losing consciousness. When you wake up, all you see before you is the massive rear end of a busty beauty who calls herself a saint…
You find yourself confused, waking up under an unfamiliar ceiling, having your penis sucked by an unknown beauty, and engaging in a six-nine, licking her pussy. But as a horny virgin, you wouldn't refuse a feast laid out before you. You indulge in the sucking and being sucked, until you reach your climax.
Through all this, you are taught by the busty saint – Mika. You are told that you are a hero summoned to another world. Your mission as a hero is to defeat the Demon King.
And that Demon King is…
Mao, who suddenly appeared in another world, wielding immense power to control monsters, changing the world's official language to Japanese, abolishing studying, ordering all chefs in the world to develop and present delicious sweets, and turning all rebels, both human and monster, into cute little kittens. She is a self-centered, arrogant, big-boobed, spoiled little sister.
"Shall we make Demon King Mao fall head over heels for your hero's penis, darling?"
Now is the time for your useless virgin penis to become a holy sword for defeating the Demon King in another world!