兄貴の妻は俺の肉妻 ～夫には見せない淫らな貌を義弟に晒す巨乳若妻～

The protagonist, Amagaya Ryouta, failed his university entrance exam and became a ronin while living with his brother and sister-in-law. Ryouta's brother, Amagaya Shuuya, is excellent in academics and appearance, making Ryouta feel inferior. Ryouta loses motivation to study and becomes a recluse at home, with Shuuya constantly lecturing him. When Shuuya's wife, Amagaya Honoka, offers to teach Ryouta, he resists due to his discomfort around her. However, he becomes aroused by her touch, making things worse. One day, Honoka's behavior changes and she starts to seduce Ryouta. He suspects something is wrong but goes along with it. After discovering that Shuuya asked Honoka to sexually satisfy Ryouta, Ryouta becomes angry and contemplates stealing Honoka from him. However, he starts to question if there is more to Shuuya's intentions and decides to investigate Honoka to uncover the truth.