The protagonist, Oda Keisuke (name changeable), is your average college student.
With his parents constantly away on business, he lives almost alone with his older stepsister, Mio (a bombshell), who works as a teacher at his school.

His first time was “taken” by Mio during a tutoring session, and ever since, he’s been under her thumb, treated more like an errand boy than a brother.
Ironically, though, all that “tutoring” paid off—his grades are excellent. These days, he’s trying to make some extra cash by offering tutoring services, posting ads on the local park bulletin board. But so far, no takers.

One day, while Mio is out, one of her students, Kobayakawa Ayano, shows up unannounced.
She’s desperate for help before a big test. Even after Keisuke explains that Mio isn’t home, Ayano looks like she’s about to cry, pleading for someone—anyone—to help her study. Feeling sorry for her, Keisuke reluctantly agrees to step in as her temporary tutor.

But once she’s inside, Ayano seems less interested in studying and more interested in grilling Keisuke about his life.
When he finally asks why she’s so curious about him, she drops a bombshell: Mio has been telling her students at school that her amazing figure is all thanks to her little brother “massaging” her every day!

Before Keisuke can process this, Ayano starts opening up about her own insecurities—literally. She exposes her chest, genuinely distressed, and begs for his help.
One thing leads to another, and before he knows it, Keisuke’s hands are all over her. And, well… Ayano doesn’t seem to mind.

“If I learn more about men, I’ll… grow even more, right?!”
As Ayano lies back on the bed, Keisuke can’t hold back anymore. He ends up taking her virginity.

A few days later, Keisuke is on edge, worried Mio might find out what happened. But then Ayano shows up again—this time with friends in tow, boasting about her “successful bust enhancement”!

From there, it’s a domino effect. Girls start lining up for Keisuke’s “tutoring,” and things escalate quickly. Before long, Mio finds out, and chaos ensues.
Amid all this, more and more girls, hearing rumors about the miraculous bust-enhancing tutor, start showing up, hoping for the same results.

What will become of Keisuke?
And will these girls’ desperate dreams of a bigger bust come true?!

[Translated from dlsite]