Seeing her younger brother unable to masturbate due to injuries to both hands, Nanami couldn't bear to watch and helped him stroke his penis. Since then, secret masturbation assistance has become a daily routine. It doesn't stop at just handjobs; it progresses to oral sex and mutual masturbation... Finally, the two reach the brink of penetration, establishing a rule: "If we only insert the tip 3cm, it doesn't count as sex." They continue their incomplete glans-only sex...