Ever since Masamune and Sagiri Izumi's parents died, Sagiri has been living in isolation despite her stepbrother having moved on with his life. They now live their lives independently of each other, but Masamune still tries to earn an income sufficient to uphold both of their lifestyles. As an aspiring light novel author, Masamune has been in the industry for over three years now. Although he has worked with his illustrator Eromanga-sensei ever since his debut, he has never met them in person. A series of events eventually lead to the realization that Eromanga-sensei's true identity is his stepsister, Sagiri! Eromanga-sensei focuses on the step-siblings as they experience hardships from being in the light novel industry and as Sagiri begins to reopen her heart to society.