Our story opens at some unknown establishment where two boys and a girl
sit at a table, their connection to each other prior to this meeting is
never really established upon but we do the reason for their
"get-to-gether". It seems Reina Toyohara (the girl) is looking for a
private tutor and so Yoshio (who seems rather upset about the whole
predicament) is to ask this of Yusuke (who happens to have no idea
what's going on). When finally the question is asked, Yusuke turns his
attention from Yoshio to Reina and is almost immediately drawn in by
the size of her massive breasts, and within seconds they are the only
thing on his mind, and like any young man, having been offered the
chance of a lifetime, he agrees to be her private tutor.
Later, presumambly that night, Yusuke shows up at her house where he meets her mother, whose breasts seem to be even bigger!
He has little time to think on such matters before he and Reina go off
to her room to do her homework. As Reina sits at a desk and works
steadily on her studies, Yusuke stands behind her peering over her
shoulder. However, it is not her work he is gazing at intently but her
boobs, and when at last he can hold it in no longer he lunges for them
and begins playing with them (while her shirt is still on) and she
cries out her dismay. That said, she never actually forcefully tries to
stop him and so things only get more intimate from there as Yusuke
further explores his infatuation with Reina's breasts. After their
"activities" commence, he returns to his own apartment and drifts to
sleep with the thoughts of what a magnificent rack both Reina and her
mother Mitsuki have...
The next day, Yusuke returns to the Toyohara household but when the door opens it is Mitsuki rather than Reina who stands at the door. Yusuke seems to have come early and as such, Reina is busy at the moment (taking a bath). Yusuke offers to go wait in her bedroom for her but Mitsuki says she has some cake and he should go to the living room where she'll bring him a piece. So he sits on the couch and she comes to sit beside him, he tries to move over and get more space between each other but she follows his movements and bends over to whisper into his ear, revealing that she knew what he and Reina did last night.
She explains that Reina fell in love with Yusuke after she saw him at the music festival, and so had Yoshio arrange their partnership as student and tutor. The conversation changes to even stranger things as Mitsuki's hand strays down to Yusuke's crotch and she speaks of how sexually frustrated she is. That's where the real action begins. It starts with a little touching, but by the end they have a full-blown session of sexual intercourse which concludes with a single threat, that this is something Yusuke can only do with her, not Reina, or she will never forgive him.
So, as you can expect, when Reina finishes her bath and Yusuke joins her in her room to do some math work, he completely drives out Mitsuki's threat from his mind and the loving begins. However, this time, it doesn't just stop at some touching, but they engage in sexual intercourse...bad timing of course cause something very wrong to happen when Mitsuki opens the door to the bedroom (bringing tea) with Yusuke inside her daughter!