Masamune Izumi is a high school student who earns his living writing light novels. He has never met the anonymous artist who illustrates his books under the pen name Eromanga-sensei, known for dependable work and wildly lewd drawings. At home he looks after his younger stepsister Sagiri, his only remaining family, who has not left her room in a year and bangs on the floor when she wants him to bring her food. Through an embarrassing series of events, Masamune learns that his illustrator has been Sagiri the whole time. As a popular female author steps in as a rival, the two work on their relationship and face the light novel industry together, in a comedy and romance with ecchi touches.