A friend's father suddenly disappears, and since he had no children, the friend ends up adopting one of a pair of twins. The boy grows up and enters university, but when applying for a passport before studying abroad, he discovers the truth about his real parents. Shaken by the revelation, he's invited by his friend on a trip to calm down and talk things through. Learning he's not biologically related to the woman who raised him, the son begins to see his adoptive mother as a woman, and new feelings start to emerge. During their stay at a hot spring, the son becomes captivated by his adoptive mother's beauty, awakening primal instincts within him. He takes Hibiki's hand and makes her stroke him, then instinctively takes her into his mouth, surrendering to pleasure beyond reason and taboo. Transcending their pseudo-parental bond, forbidden emotions arise, culminating in an illicit tale of mother-son copulation.