A Sister's All You Need.

Itsuki Hashima, a popular young novelist whose work always includes little sisters, is having difficulty coming up with a new series. One day, his younger step-brother Chihiro and some of his colleagues come over for dinner, drinks, and games.

Nayuta and Miyako walk in on Itsuki naked!? He's trying to act out the little sister character in his new volume, but he ends up embarrassing himself and complicating the trio's relationship.

Itsuki, Nayuta, and Miyako decide to escape the cold by going to Okinawa where they visit a remote island so Itsuki can do some research. Meanwhile, Nayuta decides to flail around naked in the water, and Miyako eventually joins in as well.

Nayuta, Miyako and Haruto gather at Itsuki’s room for Valentine’s Day, and Haruto tries to show off his friendship with Itsuki again. Toki then shows up with the chocolates that fans had sent to them.

Itsuki is nearing an important deadline and tries to flee from his editor, Toki, but is immediately caught. However, he manages to escape and goes to a hot springs with Setsuna.

Itsuki has a meeting with Kaiko Mikuniyama, the artist who will be doing the manga version of “Mahou Academy.” She looks very refined, which makes Itsuki nervous, but apparently she loves little sisters as much as he does.

In order to cheer up Haruto after his anime flopped, Itsuki gets him and Nayuta, Miyako and Chihiro together to play a TRPG.

Haruto reveals how he feels about Miyako, and Itsuki promises to help him out. Around the same time, Nayuta, who sees Miyako as an older sister, tells Itsuki that she wants to throw a birthday party for Miyako.

Toki brings Itsuki the manuscript for the Mahou Academy manga. Itsuki was looking forward to what Kaiko had drawn because of her art quality, but it turns out there's a problem?!

Chihiro starts working part time at Ashley’s office. Chihiro cleans Ashley’s whole office and then goes to take a bath and Ashley finds out Chihiro’s “secret.” Meanwhile, Itsuki heads to the zoo with Nayuta so he can do some research for his next book.

Itsuki asks Miyako to take something to the publisher for him and she runs into Nayuta’s editor. Miyak offers to go see Nayuta and get her to finish the manuscript in person. Miyako then realizes that she might want to be an editor herself.

Itsuki falls asleep at the kotatsu and has a dream about his middle school days. Back in modern time, Itsuki throws a party to celebrate his book becoming an anime. Haruto brings a board game where everyone gets to be a light novel artist!