Waking up, Kim Hajin finds himself in a familiar world but an unfamiliar body. A world he created himself and a story he wrote, yet never finished. He had become his novel's extra, a filler character with no importance to the story. The only clue to escaping is to stay close to the main storyline. However, he soon finds out the world isn't exactly identical to his creation.

The Novel's Extra: Cube Arc is the first arc of the web novel The Novel's Extra. The original web novel was originally released on Munpia for 479 chapters from April 1, 2018 to August 21, 2020. The Cube Arc is composed of the first 144 chapters of the web novel, and was published in print by KW Books under the Witch Book imprint (위치북) on November 1, 2021 after a successful crowdfunding campaign.

The original web novel was published digitally in English by Wuxiaworld.