The main character is just a normal high school student who spends his days with his beautiful childhood friend, Hinohara Hina. However, one day, while awaiting for summer vacation to come, his life changes when the new transfer student, young-looking but beautiful girl Unazuki Sakuya, claims him to be hers. It's soon revealed that she did so because she needs him to be her partner for a ritual where she needs to have his sperm. However, after being interrupted by his childhood friend, the two start fighting for the main character's affections, and his erotic life kicks into action.
Source: VNDB