Bored and horny, a mature sister and her younger brother find themselves alone in the house. They start masturbating separately, she fingering her wet pussy and he stroking his hardening cock. Caught in the act, they argue heatedly, but soon their desires overcome guilt and they start masturbating openly in front of each other. The sight of her brother's thick, huge cock sends shivers down her spine, and she fantasizes about feeling it inside her. Just as they're about to act on their mutual yearning, a noise from outside forces them to abruptly end their taboo encounter.