The Vegetarian tells the story of Yeong-hye, a home-maker who, one day, suddenly decides to stop eating meat after a series of dreams involving images of animal slaughter. This abstention leads her to become distanced from her family and from society. The story is told in three parts: "The Vegetarian", "Mongolian Mark", and "Flaming Trees". The first section is narrated by Yeong-hye's husband Mr. Cheong in the first person. The second section is narrated in third person focusing on Yeong-hye's brother-in-law, and the third section remains in third-person but focuses on her sister, In-hye, while sporadically speaking in the present tense.
Mr. Cheong considers his wife to be "completely unremarkable in any way". He explains that when he first met her, he was not even attracted to her and that suits him just fine. Mr. Cheong is content meandering through life; it seems as if his only goal is to live a conventional, unremarkable life. He chooses to marry his wife since he thinks she would prove to be a good, dutiful wife who would fit nicely into the kind of lifestyle he seeks. After several years of relatively normal marriage, Mr. Cheong wakes up to find his wife disposing of all meat products in the house. He demands an explanation, and Yeong-hye replies vaguely that "I had a dream." Mr. Cheong attempts to rationalize his wife's life decision over the next few months and to deal with vegetarian meals at home, but eventually calls Yeong-hye's family and an intervention is scheduled. While around the dinner table, Yeong-hye's family attempts to convince her to eat meat; her father, who served in Vietnam and is known for his stern temperament, slaps her when she refuses. Her father then asks a reluctant Mr. Cheong and Yeong-hye's brother Yeong-ho to hold her arms while he force-feeds her a piece of pork. Yeong-hye breaks away, spits out the pork, grabs a fruit knife, and slits her wrist. The incredulous family rushes her to a hospital where she recovers and where Mr. Cheong admits to himself that she has become mentally unstable. As the section ends Yeong-hye manages to walk out of the hospital and when she is tracked down, she reveals a bird in her palm, which has a "predator's bite" in it, and she asks "Have I done something wrong?"
The husband of Yeong-hye's sister In-hye, whose name remains unstated, is a video artist. He imagines a love-making scene between two people, with their bodies decorated by painted flowers and, upon learning that Yeong-hye has a birthmark shaped like a flower petal, he forms a plan to paint and record her in order to bring this artistic image to life. It is revealed that he is attracted to Yeong-hye, especially after checking up on her—the narrator reveals that Yeong-hye has been served divorce papers by Mr. Cheong—and finding her unabashedly naked in her apartment. Yeong-hye agrees to model for him and he paints flowers across her body in a studio rented from an art professor in the area. He follows up this project with a second piece of art, which involves recruiting a fellow artist to join Yeong-hye in a sexually-explicit film. When the brother-in-law asks if the two will engage in actual intercourse, his friend becomes ashamed and leaves. Yeong-hye, who had become aroused during this sequence, claims it was because of the flowers painted on the man's body. The brother-in-law asks a friend to paint flowers on him and visits Yeong-hye, where the two engage in a recorded moment of intercourse. When his wife discovers the film, she calls "emergency services", claiming that both he and Yeong-hye are mentally unwell. He contemplates jumping off of the balcony, most likely to his death, but remains "rooted to the spot" and is escorted out of the building by the authorities.
In-hye remains the only member of the family to support Yeong-hye after her mental and physical decline. She has separated from her husband after the events of the previous section, and is left to take care of their son in addition to her deteriorating sister. As Yeong-hye's behavior worsens, she is admitted to a mental hospital at Mount Ch'ukseong, where, despite receiving high-level treatment for mania, she behaves gradually more plant-like. On one occasion she escapes the hospital and is found standing in a forest "soaked with rain as if she herself were one of the glistening trees". In-hye, who constantly ruminates about the pain of dealing with her divorce and the care of her child and who throughout the chapter shows signs of her own depression and mental instability, visits Yeong-hye regularly and continues to try to get her to eat. Yeong-hye has given up food altogether, and when In-hye witnesses the doctors force-feeding her and threatening sedation to prevent vomiting, In-hye bites the nurse holding her back and grabs her sister. In-hye and Yeong-hye are driven to a different hospital by ambulance, and In-hye observes trees as they pass by.