Masamune Izumi is a high school student who writes light novels. Sagiri Izumi is a shut-in who never leaves her room. A year ago, she became Masamune’s little sister. But one day, Masamune discovers a shocking twist—The artist "Eromanga Sensei," who has been drawing illustrations for his novels, is none other than his little sister Sagiri! His cute little shut-in of a sister, who has been living under his roof, is using an obscene pen name and draws lewd illustrations?!