Yui and Naoya, two months into dating, are not yet accustomed to being a couple. A man passing by in front of them calls out. "Yui-chan?" At that moment, Yui was speechless. Because the person standing there was Yui's first sexual partner. Yui's past: before she and Naoya started dating, she had sex with a man out of curiosity about sex. The man threatens Yui, saying he will expose her past act to her boyfriend. And once again, secrets Yui can't tell her boyfriend begin to accumulate within her...