One afternoon on a holiday --
Our protagonist, Mitsui Shuusuke, has left his parents' home and is living alone while attending university. His younger sister, Mitsui Hina, suddenly shows up for a visit.
When Shusuke asks what's wrong, Hina hugs him and, crying, tells him that out of curiosity, she used a magic spell she learned on the internet, and her breasts grew larger.
How can she cancel the effects of the magic... "Only my big brother i can ask to do something like this for me..." Hina said, looking depressed.
After researching the magic Hina used, he finds out that her breasts are growing as a side effect of the magic, and that drinking the semen will cure the side effect.
Hina is confused as she learns what she has to do next, but it's to restore her breasts to their original state, and more importantly, the semen belongs to her brother...
Hina blushes as she mutters these words, and Shusuke understands her feelings as he picks her up and leads her to bed...