In the world of high school, Masamune Izumi is known as both a student and a writer of light novels. His sister, Sagiri, has been secluded for a year and relies on Masamune to bring her meals. As the only family to one another, Masamune strives to help Sagiri break free from her isolation. Masamune's illustrator, who goes by the name of "Ero Manga Sensei," is skilled despite their crude drawings. Masamune had always believed that the illustrator was simply a devoted otaku, but it turns out that the creator of the drawings is Sagiri herself! As if their relationship wasn't complicated enough, a successful Shoujo manga artist appears as their competition. This peculiar tale of a "new sibling romantic comedy" is one that is sure to leave readers entertained.