Although they are brother and sister, they have always been instinctively attracted to each other. However, this is a faint love that will not come true. Before they know it, they become aware that they are a man and a woman, but each of them uses their rationality to refrain from interfering with each other. However, on his brother's 20th birthday, his parents reveal that the two are actually step-siblings. My sister happened to hear about it. Late at night, after she was asleep, her sister came alone to her room... Mutual feelings that were suppressed until now explode! They intensely seek each other out through intense sex that exposes their romantic feelings!