Masamune Izumi is a high school student and a light novel author who writes under the pen name "Masamune Izumi." After the death of his stepmother, he becomes the primary caregiver for his younger stepsister, Sagiri Izumi, who has been a shut-in for over a year, refusing to leave her room. Masamune struggles to connect with Sagiri, who spends her days immersed in anime and manga, until he discovers that she is the anonymous illustrator "Eromanga Sensei," who has been providing the artwork for his novels. This revelation forces the siblings to confront their strained relationship and work together professionally.

As Masamune and Sagiri begin collaborating more closely, their dynamic shifts from distant to increasingly interdependent. Sagiri’s talent as an illustrator grows, and she becomes more involved in the creative process, while Masamune navigates the challenges of balancing his writing career with his responsibilities as a caregiver. Their relationship is further complicated by the arrival of other characters, including Elf Yamada, a rival light novel author who becomes infatuated with Masamune, and Muramasa Senju, a talented writer who challenges Masamune’s skills. These interactions create a mix of professional rivalry and personal tension, as each character seeks to carve out their place in the competitive world of light novels.

The series explores themes of family, creativity, and personal growth, with a focus on the evolving bond between Masamune and Sagiri. While their relationship is central to the narrative, the story also delves into the broader light novel industry, highlighting the pressures and expectations faced by creators. The characters’ interactions are often humorous and lighthearted, but they also address deeper issues such as isolation, self-doubt, and the search for identity. Through their collaboration, Masamune and Sagiri gradually overcome their individual struggles, finding strength in their shared passion for storytelling and art.