Graduate student Rei visits her long-lost older sister Makoto, whom she hasn't seen in twenty years. Based on her research theme of "interpersonal communication and exchange among living beings," Rei becomes interested in engaging in lesbian exchange with Makoto, gradually developing a deeper relationship. Their lesbian encounter begins with passionate, saliva-dripping kisses, progressing slowly into breast sucking and cunnilingus. As the experience unfolds, Rei becomes increasingly obsessed with the vulgar and intense lesbian intimacy, losing herself completely in body and mind, drawn deeper into its allure.