DASS-900 | After showering my mother with love every day for a month, she lost all self-control, and we had sex over and over again to conceive a child. - Hibiki Otsuki

In the Otsuki household, the father, Yoshio, is assigned to work overseas for an extended period...
The mother, Hibiki, and her two sons (Haruki and Yuma) are left to stay at home. As their father remains away, the eldest son, Haruki, vows to "protect his mother" and directs his strong feelings towards her, confessing his love for her for a month... His younger brother, Yuma, witnesses this and has complicated feelings, eventually realizing his own feelings for his mother. The brothers clash with their respective feelings, compete for their mother's affection, reconcile with each other, understand each other, and yearn for one another.