The protagonist is a middle-aged doctor at an obstetrics and gynecology clinic.
His patients are mostly ordinary pregnant women, young moms, and middle-aged women dealing with menopause.
Rarely, if ever, does he get to see the kind of young patients he prefers.

Then, one day, a high school girl shows up—a rare sight.
She’s wearing the uniform of a prestigious school, one you don’t see around often.
It seems she chose this slightly out-of-the-way clinic, not exactly popular for its “obstetrics and gynecology” services, to avoid being noticed.
She arrives a little after hours, just as the nurses are starting to pack up and leave.
The girl has long black hair and looks pure, innocent—exactly the type the protagonist likes.

“For the past two or three months, my periods have been irregular… Is something wrong?”

It’s nothing serious.
Just the hormonal imbalance typical of adolescence.
A dose of hormones would fix it right up.

But then, the beast within the protagonist begins to stir.
He's about to explode from the stress and pent-up desires he’s been holding back.
At the moment, it’s just him and this high school girl in the clinic.

“Well then… you know what I’ve got to do, right?”

[Translated from dlsite]