June, the rainy season. Kei, a fourth-year college student, is at a major crossroads. Since the day of the fireworks display when he interrupted the confession from his younger sister Haruka, the relationship between them has gone cold, and still remains in this way for three years. To become truly happy together, until the "Answer" is found, there is no way to respond to his sister's feelings. While desperately holding back the feelings for his sister, Kei continues to act as a "good older brother". On the other hand, Haruka is acting coldly towards her brother, who does not respond to her feelings, despite being aware of each other. As the rainy season continues, Kei's worries deepen. To become truly happy together. If he can find the "Answer", he should be able to respond to his sister's feelings. One day, as a result of consulting with his friend Daichi, Kei reluctantly goes to a hostess club, since there is a girl whom his friend really wants him to meet. Appearing in front of Kei is a girl who could have been mistaken for his beloved sister, a face that looks exactly like Haruka. The girl introduces herself as "Mitsuki", feeling "Similar", the two of them become intensely attracted to each other. With the appearance of "Mitsuki", the relationship between the siblings "Kei" and "Haruka" begins to change drastically.