Time Hippopotamus, an original anime series by Musashino Animation, faces cancellation due to financial constraints from its sponsor, leading to significant staff departures, including the resignation of Masato Marukawa, the studio's president. In April 2019, amidst this turmoil, Aoi Miyamori and the remaining team members catch an episode of Third Aerial Girls Squad Medusa☆, an ecchi sequel developed by Studio Taitanic Film. The new president, Shun Watanabe, engages in discussion with Miyamori about her potential role in producing Air Amphibious Assault Ship SIVA, an original feature-length anime set to debut in February 2020. Battling challenges from Gōtarō Katsuragi, who urges Miyamori to complete the film after GPU, the attached studio, fails to fulfill its storyboard obligations while requesting a larger budget, she finds motivation in Marukawa's legacy and ultimately agrees to take on the project.
As production kicks off just nine months ahead of the film's release, Miyamori devises a strategy to incorporate the materials from Time Hippopotamus, persuading Seiichi Kinoshita to come onboard as director. Their collaboration leads them to Shimeji Maitake](/actor/asami-takano), who takes on the task of writing the script. Miyamori reaches out to Yumi Iguchi for character design assistance, and enlists Rinko Osagawara's support. Misato Segawa and Yūichiro Shimoyanagi try to bring in Ryōsuke Endō—heavily impacted by the prior show's cancellation. Encouraged by his wife Mayumi, he ultimately decides to join the crew. Ema Yasuhara is brought into the fold, while Shizuka Sasaki auditions for a role, and Misa Tōdō accepts the crucial position of chief animator. In exchange for a favor, Miyamori arranges for Shigeru Sugie to assist with animal drawings, allowing her to help with his animation class for children.
With only six months to go before the premiere, Maitake expresses dissatisfaction with his script. However, after watching Sasaki's audition and gathering inspiration from Midori Imai, he manages to complete it. Meanwhile, Masashi Yamada faces false accusations and Yasuhara encounters difficulties with her corrected keyframes. Even from different studios, Tarō Takanashi and Daisuke Hiroaka extend their assistance to the film. Two months later, Erika Yano returns to Musashino, providing support with Kinoshita's storyboarding efforts and helping to recruit Hiroshi Iketani. In the meantime, Miyamori, Yasuhara, Sakaki, Tōdō, and Imai join Sugie's animation class, gaining invaluable experience and inspiration. Eventually, Sasaki secures the role of Arte in the film, and Yasuhara successfully submits her polished keyframes.
Tensions rise when Katsuragi alerts Miyamori and Kaede Miyai about GPU President Yoshio Misaki's assertion that his studio claims the rights to the film. Concerned that GPU may try to pause production or impose a joint production arrangement, Miyamori and Miyai confront Misaki about his contract violations. As the premiere approaches with just three weeks remaining, the film nears completion, although Kinoshita expresses concerns over a rushed ending. Miyamori urges the production team to refine the final scenes to elevate the film's quality for both their satisfaction and the audience's enjoyment. Finally, on opening night, Miyamori, Yasuhara, Sakaki, Tōdō, and Imai gather at a theater to witness the culmination of their hard work as they watch the finished feature on the big screen.
