Tougetsu Umidori is a high school girl with a strange compulsion: she steals pencils from her classmate, Yoshino Nara, shaves them, and eats the shavings over rice. This peculiar habit is only the surface of a deeper, more troubling condition—Umidori is incapable of lying, a curse that has left her isolated and unable to form close friendships. She lives alone in an apartment, her parents divorced, and maintains a careful distance from everyone, including Nara, who considers her a friend.

The story begins when Nara, whose face is oddly expressionless despite her normal voice, confides in Umidori about the pencil thief in their class. Umidori manages to avoid confession through non-committal answers, but that evening, her life takes a sharp turn into the surreal. A mysterious girl in a cat hoodie arrives at her home, wielding a knife and calling herself BS or Bullshit-chan. Claiming to be a Damsel Defender sent to punish Umidori for her stalker-like crimes, she attacks. However, after a desperate struggle, Umidori learns this is all a lie.

BS-chan is not an enforcer but a supernatural being—a lie that has manifested in physical form. She feeds on other lies to survive, and with only a week left before she vanishes, she needs Umidori's help to hunt them down. In exchange for her aid, BS-chan makes an offer Umidori cannot refuse: the ability to lie, freeing her from the curse of brutal honesty that has ruined her relationships. Without fully understanding why, Umidori agrees to join her quest to kill the lies.

The series is set in contemporary Japan, shifting between Umidori's high school and her apartment, which becomes the central stage for bizarre, dialogue-driven encounters. The narrative is fast-paced and intentionally erratic, blending absurd comedy, supernatural action, and philosophical musings on truth and deception. The first volume introduces other manifested lies and explores Umidori's shared past with Nara, revealing that Umidori's unintentionally truthful words once saved her friend from a dark fate.

In the second volume, the story grows even stranger. Umidori, now relatively content and partnered with BS-chan, receives a phone call from someone who claims to know her intimately—someone she buried in a grave behind her apartment complex. Meanwhile, the pencils she ate gain sentience, emerging as a loving, human-shaped entity named Togari Tsukishigaoka, leading to an even more complicated and co-dependent situation. The volumes continue to explore the nature of lies and their psychological toll, featuring a cast of eccentric and often dangerous characters, including new threats like a mysterious blonde lady.