Ever since the death of her parents, Sagiri Izumi has isolated herself from everyone, including her stepbrother Masamune. Masamune's life continues as usual, but he still tries his best to make Sagiri leave her room.

Aside from being a high school student, Masamune is also a light novel author. Although they have worked together for three years, Masamune has never met his illustrator "Eromanga-sensei." Through a chain of awkward events, he ultimately realizes that Eromanga-sensei has been his stepsister all along!

Eromanga-sensei follows the two step-siblings—who experience hardships as part of the light novel industry—and looks at their lives as Sagiri begins to reopen her heart to society.