The protagonist, Yeong-hye, is described by her husband, Mr. Cheong, as "completely unremarkable in any way." Though the description may seem negative, this trait is in fact what makes him content in his marriage, as he expresses a desire to pursue a life that is itself unremarkable. Mr. Cheong continues on to narrate part one, simply named "The Vegetarian," save for a few asides where Yeong-hye interrupts the narrative to recount her nightmares involving scenes of animal slaughter, the same nightmares that inspired her to throw hundreds of dollars worth of meat away from their fridge and freezer to pursue her new diet.
Part one goes on to describe Yeong-hye's seemingly bizarre behavior, such as her refusal to wear a bra in public or actively engage in conversation at a business dinner hosted by Mr. Cheong's boss; an act that breaks Korean societal norms for women and causes Mr. Cheong to "lose face." Mr. Cheong begins to become more frustrated and aggressive towards his wife, eventually culminating in sexual violence. After some time of Yeong-hye living as a vegetarian, her extreme weight loss causes Mr. Cheong to tell her family of her new diet, which incites extreme concern in her parents especially. Her mother then decides to cook a large meal of Yeong-hye's favorite meat-filled dishes at their next family gathering. In the penultimate scene of part one, Yeong-hye's father, described as a stubborn man with a quick temper, attempts to force-feed her pork and then strikes her at her repeated refusal. Yeong-hye brandishes a knife at her family members before slashing her own wrists and is subsequently rushed to the hospital.
Part two, "Mongolian Mark," takes place two years after Yeong-hye's transport to the hospital and is narrated by her brother-in-law. He is never given a name, but we learn he is a video artist. After offhandedly learning that Yeong-hye (his wife, In-hye's, younger sister) still has a Mongolian Mark, he begins to fantasize about a love-making scene between two people whose bodies are covered in painted flowers due to the fact that the Mongolian Mark is described as a blue petal-shaped birthmark that does not typically remain on the body into adulthood.
Part two opens with a scene where Yeong-hye's brother-in-law steals away to see an erotic dance performance with performers adorned in painted flowers, but the show ultimately fails to actualize his vision. He eventually realizes his only option to fulfill his artistic pursuit is to convince Yeong-hye to be a part of his video project while also coming to terms with his physical attraction towards her. Though he successfully convinces her to be a part of the erotic video project, his plan to include his fellow artist friend, J, in the video falls through when he instructs J to have sex with Yeong-hye during filming. Yeong-hye's brother-in-law eventually succumbs to his desires and participates as her counterpart in the video himself. Part two ends in a scene where In-hye discovers her husband in bed with her sister the morning after filming the video, and In-hye subsequently calls emergency services on them.
In-hye narrates the third and final part of the novel, "Flaming Trees," which takes place one year after the events of "Mongolian Mark," though it often flashes back to recount her side of the story of her husband's absence and their failed marriage. She explains that Yeong-hye's vegetarianism has spiraled into mental illness, and now abstains not just from eating meat, but from most forms of human interaction. After the brutal force-feeding attempt by their father, Yeong-hye was hospitalized.
Despite receiving specialized treatment, she maintained her belief that the only way to avoid the brutality that humans inflict upon each other and animals is to become a plant. This is what compelled her to accept her brother-in-law's proposal to be part of his erotic video project, and she adopted a fondness for having the flowers painted on her body. We also discover that her brother-in-law attempted to commit suicide by jumping from her apartment's balcony when In-hye called emergency services on them. He remains mostly absent from the remainder of the novel save for an attempt to see their son, Ji-woo.
Mr. Cheong also became an absent character after Yeong-hye's initial hospitalization, and her parents and brother (never given much attention in the narrative) slowly distance themselves from Yeong-hye as well, leaving In-hye as her sole source of support. In-hye periodically visits her sister at Ch'ukseong Psychiatric Hospital in hopes that she will eventually recover from what is claimed to be intense mania. Towards the end of the novel, however, Yeong-hye claims that she is no longer animal, but rather, a plant. She becomes non-verbal, and stops eating all together. After a final attempt by the nurses at Ch'ukseong to force-feed her, In-hye, witnessing the scene unfold, bites the arm of a nurse and both are sent to another hospital by ambulance. The novel closes with and ambiguous scene in which In-hye gazes through the window at the trees rush by.