In 1988, Japan enacts the Media Betterment Act, a law aimed at curbing freedom of speech in media publications because the government views certain content as a bad influence on the public. The enforcing authority, the Media Betterment Committee, is empowered to crack down on publications they deem dangerous. Fast forward to 2004, when Iwao Nishina, a survivor of the 1999 Hino Library incident, takes the helm to combat censorship by founding the Library Defense Force.
In a quiet bookstore, Iku Kasahara, a high school student, witnesses the confiscation of books, including her beloved fairy tale. When she resists handing over the volumes, she is rescued by a mysterious member of the Library Defense Force who proclaims that the books are now under their protection. This bold moment fuels Kasahara’s decision to join the Library Defense Force in 2019, driven by the hope of finding her own "prince." She is placed under the supervision of Atsushi Dōjō, an instructor with whom she initially clashes, setting the stage for a difficult but important mentor–mentee dynamic.
Soon, Kasahara and Hikaru Tezuka are moved to the elite Library Task Force, led by its squad leader Ryusuke Genda. On their return from a training camp, the unit passes by a bookstore where Kasahara instinctively intervenes in another censorship operation, insisting that the books remain protected by the Library Defense Force, even though her rank makes it hard to seize them. Dojo steps in to protect her, but the incident earns Kasahara a reprimand for her reckless action.
The conflict intensifies when the Media Betterment Committee demands that the Library Defense Force surrender the graphic novels housed at the Musashino Library as part of an active murder investigation that suggests those books may have inspired the crime. The Library Defense Force refuses, prompting a confrontation with the committee. Amid the fighting outside, Kasahara’s friend, library staffer Asako Shibasaki, discovers two MB agents inside. Kasahara and Tezuka clash with the intruders and manage to recover the confiscated books, ending the assault after about an hour as agreed. In the aftermath, Tezuka asks Kasahara to be his girlfriend, but she does not provide an answer yet.
Sometime later, industrialist Sohachi Nobeyama—a museum owner in Odawara housing documents about the Act’s controversial passing—passes away. In his will, all materials from the museum are to be transferred to the Musashino Library. The Library Defense Force girds for a new defense of the museum against a MB assault. Squad leader [Ryusuke Genda] dismisses Kasahara from the task force and assigns her to guard Iwao Nishina during Nobeyama’s funeral. Kasahara and Nishina are subsequently kidnapped by MB supporters who demand the Library Defense Force burn the museum materials. Kasahara is allowed to speak to confirm their safety and hints that the captors are somewhere in Tachikawa. Dojo, aided by Mikihisa Komaki, rescues Kasahara and Nishina. The hostage crisis and the museum assault are reported by journalist Maki Orikuchi, a friend of Nishina. In the end, Kasahara rejects Tezuka and recognizes that her true “prince” is Dojo.
