New teacher Ukima Sousuke is assigned to Kamomiya Academy and is passionate about his dream job as a teacher.
But at home, Sousuke is also the older brother of three younger sisters.
Himari, the eldest, is a soft-spoken, airhead.
Yuzuki, the second, is reliable and clingy.
Mio, the third, is a devilish teaser.
The three younger sisters, who have been attached to their older brother since childhood, all enrolled at his school.

When the brother starts working as a teacher, Sousuke takes advantage of the situation and becomes a siscon, clinging to them from the time he wakes up to the time he goes to and from school, and even around the school at all times.
However, perhaps all of their growth has been focused on their breasts, as while they haven't grown in height, their breasts have grown into an incredibly large G-cup.

As a result, all of his siscon behavior, which has remained unchanged since childhood, is now fueling his desires as a man.
As he grapples with his complicated feelings toward his younger sisters as both a brother and a man, one day, the school festival approaches.

Sousuke stayed behind after school when his younger sister Himari came in with tears in her eyes.
"I... I've started producing milk! What, what, what should I do, Onii-chan..."

Sousuke tried to calm her down, but was at a loss as to what to do. Himari timidly spoke up.

"Um, you see... Onii-chan... Um, can you... Squeeze my breasts... Is that okay?"

Even if she's your sister, massaging a woman's breasts and squeezing breast milk out of her... There's no way a teacher would allow that!

Sousuke tried to restrain himself with common sense and reason, but Himari pressed her
G-cup breasts together and looked up at him, pleading.
"Please, Onii-chan... I ask you to do this... You're the only one I can ask..."

Those words instantly broke Sousuke's reason, and he listened to his sister's request and agreed to squeeze her breast milk.

She has a cute appearance and is on the short side. And her breasts are a G cup.
How long will Sousuke be able to keep his sanity as he squeezes breast milk at the request of his sisters...?