Tougetsu Umidori is a high school girl with an unusual and compulsive habit. For over a year, she has been secretly stealing pencils from her classmate and friend, Yoshino Nara. Her reason is not malice but a strange craving: she shaves the graphite from the pencils and eats it over rice, driven by an intense desire to consume the fingerprints left on them by Nara. Umidori operates under a significant personal limitation, as she is unable to tell a lie, a condition that has socially isolated her since childhood and made her a target of ridicule. To avoid confrontation, she carefully navigates conversations with partial truths and non-committal answers. When Nara, who treasures her writing instruments, asks for Umidoris help in finding the mysterious pencil thief, Umidori finds herself in a precarious position, forced to use evasive language to protect her secret.

The situation escalates dramatically when a strange girl wearing a cat hoodie appears at Umidoris door, calling herself Bullshit-chan. The intruder claims to be a Damsel Defender sent to kill Umidori for her perceived stalker-like actions against Nara. After a desperate struggle in which Umidori overpowers her, the truth is revealed to be far stranger than a simple assassination attempt. Bullshit-chan is not an assassin but a physical manifestation of a lie. Abandoned by the person who created her, she must consume other living lies to avoid disappearing from existence. With only one week left before she fades away, she has come to Umidori seeking help to hunt down a powerful manifested lie. In exchange for her assistance, Bullshit-chan promises to grant Umidori the one thing she has always lacked: the ability to lie.

The first target of this bizarre partnership is none other than Nara Yoshino, who herself is a living lie. Nara had once told a lie about being the most beautiful girl in the world with such conviction that it manifested into reality. This lie made her so unnaturally beautiful that she lost her job as a model because her presence overshadowed everything else. Umidori and Bullshit-chan set out to confront Nara, not to kill her, but to feed on the lie she embodies. Through their conversation, Umidori learns the true loneliness behind her friends beauty, revealing a more human and vulnerable side to the pencil-obsessed girl. This encounter solidifies the unlikely alliance, as Umidori, Bullshit-chan, and eventually Nara begin their quest to eliminate other dangerous lies that have gained physical form and the power to warp reality.

The story is set in contemporary Japan, revolving around the characters daily lives at school and Umidoris home. The narrative is driven by dense, dialogue-heavy conversations that weave together absurd comedy, wordplay, and sudden dramatic turns. The conflict stems from the need to hunt down beings called Beliars, lies that have become sentient and are using their abilities to rewrite the world in small but significant ways. A later arc, for instance, involves a Beliar using its powers to fraudulently claim that the local specialties of Kobe were invented in a neighboring town. To combat this, Bullshit-chan uses her online video channel to run a food stall at a local festival, hoping to lure out the culinary fraud. These confrontations force Umidori to confront her own inability to deceive while she helps Bullshit-chan consume these reality-altering falsehoods to sustain her own precarious existence. The series follows this unconventional trio as they navigate friendship, personal trauma, and the dangerous metaphysical ecology of lies made flesh.