Far in the future, the lifeforms known as Gadoll suddenly arose as a threat to humanity. The last surviving humans on Earth confine themselves to the Tank, a lower district in the giant mobile fortress Deca-Dence. While the Gears who live on the upper floors are warriors who go out to fight as part of the Power, most Tankers are content to provide support from the backlines, butchering Gadoll meat and reinforcing defenses. Natsume is among those who would rather go to the front lines; undeterred by her prosthetic right arm, she seeks to join the small number of Tanker soldiers who join the Gears in combat. But despite her peers at the orphanage each receiving their work assignments, Natsume’s enlistment to the Power remains unapproved. In the meantime, she begins a job as a cleaner in an armor repair team led by the hard-nosed and apathetic Kaburagi, who seems to be more than he lets on. Though initially cold to his idealistic subordinate, he soon recognizes in her the potential to upset the status quo of the world. As Natsume’s new mentor, Kaburagi prepares her for the special and unique role as a game-changing bug in the system.

Filled with an overwhelming sense of wonder for the world around her, Mari Tamaki has always dreamt of what lies beyond the reaches of the universe. However, despite harboring such large aspirations on the inside, her fear of the unknown and anxiety over her own possible limitations have always held her back from chasing them. But now, in her second year of high school, Mari is more determined than ever to not let any more of her youth go to waste. Still, her fear continues to prevent her from taking that ambitious step forward—that is, until she has a chance encounter with a girl who has grand dreams of her own. Spurred by her mother's disappearance, Shirase Kobuchizawa has been working hard to fund her trip to Antarctica. Despite facing doubt and ridicule from virtually everyone, Shirase is determined to embark on this expedition to search for her mother in a place further than the universe itself. Inspired by Shirase's resolve, Mari jumps at the chance to join her. Soon, their efforts attract the attention of the bubbly Hinata Miyake, who is eager to stand out, and Yuzuki Shiraishi, a polite girl from a high class background. Together, the four spirited girls set sail toward the frozen south, all in search of something great.