Le jeune lycéen Masamune Izumi est un auteur de light novel qui travaille avec un dessinateur pour ses illustrations érotiques, mais Masamune ne connaît pas son dessinateur, il connaît uniquement son surnom, "Ero Manga Sensei"

La vie de Masamune est toujours bouleversé par sa petit sœur, Sagiri, toujours là pour le déconcentrer.

Un jour, il découvre que son dessinateur "Ero Manga Sensei" n’est autre que sa petite sœur, et comme si cela ne suffisait pas, leur plus grand concurrent est une magnifique auteur de manga shôjo populaire.

The Izumi House’s empty mansion next door has rumors of being cursed because of continuous misfortunes. But what happens when Sagiri starts hearing piano music coming from the supposed empty mansion?! And how will Masamune handle this Fallen Master?

After observing the younger and more popular author, Elf Yamada, work on her novel, Masamune and Elf become a little closer. Sagiri notices this, and gets suspicious of their relationship, despite Masamune saying that he and Yamada-san don’t get along at all. Regardless of their argument over this matter, Masamune begins working on his “ultimate light novel”!

Motivated by Masamune’s manuscript, Sagiri puts effort into getting a little closer to Masamune, despite the awkwardness. Both of them are now working towards making the manuscript into a book, starting with the project proposal. In the midst of all this, Elf sends an emergency text message to Masamune!?

To find a clue for becoming friends with Sagiri, Megumi tries to gain an affinity for light-novels. Masamune suggests dropping by and asking if she could borrow books from Sagiri. At first, Sagiri vehemently rejects the encounter, but later presents Masamune with a certain condition…

Masamune, who is sparing no effort to publish his new title faster, immediately signs up for the light-novel competition, “Light-novel Tenkaichi Budoukai.” However, the super-popular author, Senju Muramasa, also signs up for the competition and declares to Masamune that she will crush his pointless dream! Why would she even do that?

Sagiri finds out what happened between Masamune and Muramasa. It’s not like Sagiri was happy about it or anything. In the midst of this, it’s crunch time at the editorial office as Masamune begins to turn his short story into a long novel. Elf-sensei also suggests having a wrap party at Masamune’s house as they celebrate summer in yukata outfits!

"I’ll call it the 'Summer Data Collection and Writing Camp'!" As a result of one of Elf’s ideas, the light-novel authors end up heading to a vacation house on an island in the “South Seas”. How far can Elf push the "Data Collection" card?

At the Yamada Family’s vacation house, Muramasa enthusiastically reads Masamune’s “shelved” stories. Even Masamune can’t hide his happiness when he sees someone reading his novels so passionately. However, within Muramasa’s heart filled with unhidden love towards Masamune, there was indeed a change occurring, and so Muramasa makes a certain decision.

Masamune comes home from his trip, and Sagiri is happy to see him return. They now await their new novel to go on sale. On the day of, an “Izumi Masamune Faire” is held in a store in Akihabara. While Masamune and Sagiri “explore” the area, they share their stories of how they got into writing novels and illustrating.

Masamune and Sagiri’s new work, "Sekaimo: The World’s Cutest Little Sister" is now on sale. Sagiri continues to stream from her room wearing a mask as always, and declares the curtain rising on a "Festival".