Hinata, frustrated with her busy husband, masturbates while fantasizing about her brother-in-law's underwear. Meanwhile, her brother-in-law masturbates while smelling her sister-in-law's huge bra. Witnessing her brother-in-law masturbating, the sister-in-law opens the forbidden door in Kokoro. Hinata seduces her brother-in-law while her husband is away, and their relationship gradually grows into a forbidden one, and the relationship between the two deepens with each passing day. Awakened to her sexual desire, Hinata encourages her brother-in-law to bring over a friend. Hinata ends up having a relationship with her brother-in-law's friend...