R-15 is about a boy, Taketo Akutagawa, who attends a school for geniuses: Inspiration Academy Private High School. Taketo is a genius novelist and writes erotica. Despite negative perceptions many people have of him, he aims to be at the top of his class and be recognized as the world's greatest writer.

It's the end of June, and the summer seems to be locked in place. Tomohiro Izumi, recognized by himself and those around him as an introvert, is spending another quiet day in his little corner of his classroom... or so he thought. "Congratulations! You've been selected as our next student council president!" The student council president lottery, held on the spur of the moment by the academy. Tomohiro manages to be the one randomly selected thanks to his ridiculous luck! Now the spotlight is suddenly upon him, and as a result, rumors about him begin to circulate. With no way out, Tomohiro relies on his advisor to start gathering allies he can rely on. But none of the candidates his advisor suggests, from his classmate Kano to his underclassman Asumi, and even his little sister Hiyori, are even coming to the campus!? Amidst all of this chaos, even the former president that no one could ever get ahold of, Shio, shows up...! Is it fate, or is it a foregone conclusion? The unheard of and unprecedented battles of the "Random Lottery Student Council President" are about to begin!

Takumi Tsuzuki is a high school student who attends the Umenomori Private Academy, free of charge, alongside Fumino Serizawa, a childhood friend of his whom always says the opposite of what she feels. He spends most of his time at school fending off Chise Umenomori, the granddaughter of the board chairman and a pampered princess, who is constantly roping him into her eccentric hobbies. After school, he goes to work at the "Stray Cats" confectionery, a cake shop run by his adoptive older sister, Otome Tsuzuki, until it's time to go to bed. This is the average routine in the day and the life of Takumi. Mayoi Neko Overrun follows another seemingly average day in the life of Takumi. With his sister away from the shop, having gone to save someone else in need of help, Fumino takes it upon herself to wake him up so that he won't be late for their usual walk to school together, giving him a glimpse of her blue and white striped panties in the process. What a nice way to start the day. When Otome returns home, she brings with her a girl named Nozomi Kiriya, whose hair and mannerisms resemble that of a large cat. It turns out that she is a runaway that Otome can't help but take in. Takumi's ordinary days are transformed into splendid chaos as he tries to unravel who this mysterious beauty is and what she's running away from...

The Goddess of the Machine God descended to earth to retrieve the hand of God, broke the rules and changed her fate against the will of heaven, and experienced an unparalleled love affair, achieving a double harvest in career and love. Let's see how the heroine who got the script of the hot-blooded boy comic uses the machine skills to counterattack all the way, which is exciting and full of laughter.