"Am I cumming right now? I don't underst- but, but, it feels so good." Even though we're cousins, even though I'm like a big sister to him. This younger boy's big dick is making me cum again and again. To cut his commuting time, Hinano Mitsuki, who's preparing for his entrance exams next year, found himself living together with his older female cousin Tsukasa Miyoshi, whom he calls big sis and whom he has unrequited feelings for. There's one such day when Hinano, who's doing housework in place of his slacker cousin, found one of her shirts that's gently filled with her womanly fragrance; "Tsukasa-san." He gave in to temptation, holding her shirt in one hand while getting himself off with the other. Right in the middle of this, she herself walks in on him? "I understand that it's normal for you to masturbate, you're a healthy boy, after all!" "But that! What's that you've got?! That's my shirt, isn't it?" Hinano makes a big reveal to Tsukasa, who's red and shaking with anger: "The truth is, I just can't get off to anyone else but you." At first, Tsukasa was bewildered, but gradually her mischievous spirit began to take over. "Then prove it. That wasn't a lie, was it?" "Big sis will stay here with you, so continue what you were doing before. You can do it, right?" Right beside him, Tsukasa reaches to touch Hinano's dick. He tries to refuse but the touch of her bare hand just feels much, much better than he ever imagined. Ever since that time, Tsukasa has gotten Hinano to offer his dick to her at any time with the intention of practicing techniques that she could use to make a man fall for her. I'm glad, and this feels good, but isn't all this simply for the sake of "other men"? There's no way he'll keep accepting such a twisted relationship. "Just what kind of relationship do I have with her?" Hinano lets these feelings that he's been holding back finally explode, and he ends up taking those feelings out on her body. Tsukasa, who, after all this time, has been putting on airs about being the older one, is unable to resist Hinano's sudden change. "Don't treat my clit like a dick, it's embarrassing. No, stop. Even though I just came, you keep on flicking it." Tsukasa keeps cumming over and over to Hinano, who doesn't know when to stop his counterattack. But ultimately, under the surface, she notices the screams of his pained heart. What is the final destination of this rich, sweet love between two awkward cousins who've come together under one roof?