The protagonist returns home to the countryside to visit his parents and meets again with cousin Umi in ABW-340.
He quickly notices that the lil’ cousin is now a seriously hot teenager (18+), and he even gets the chance to get a quick sneak peek at her naked body while taking a bath. This made him so horny that he began to fap in the bed thinking of her. Umi finds out about it, and after realizing that cousin is still a virgin dude, she takes complete control of the situation and playfully orders him to continue masturbating.
The cherry popping happened the next morning. And a full-blown cousin affair followed. Yatsugake Umi has an unholy perfect body. She’s too good.