Maren, a sixteen-year-old girl living in the United States in the late 1990s, wakes up one morning and finds that her mother, Janelle, has abandoned her. Before leaving, Janelle writes a brief note, which says that she loves Maren but “can’t do this anymore.” Though Maren is distraught, she understands her mother’s choice. She explains that she is an “eater,” or in other words, a compulsive cannibal. As an infant, she murdered and partially cannibalized her babysitter, Penny Wilson. At first Janelle does not believe that the infant Maren was responsible, though she comes to understand her daughter’s true nature after she similarly cannibalizes a boy named Luke Vanderwall at summer camp at age eight. The death of Luke marks the first of many times that Janelle has to quickly uproot their lives and move somewhere new in order to protect Maren from the consequences of her actions. Except for Penny, all of the people who Maren cannibalizes are boys who consider her attractive and get too close to her. At one point, Maren eats the son of Janelle’s colleague at a Christmas party, forcing a distraught Janelle to leave a well-paying job in a law-office.  

Janelle leaves Maren some money and her birth certificate, which reveals the name of Maren’s father: Frank Yearly. Though Maren thinks that her mother intended for her to find her father in Sandhorn, Minnesota, Maren decides to search out her mother first, hoping to gain a sense of closure and to determine if her mother ever truly loved her. Using the money, she purchases a bus ticket and travels to the home of her maternal grandparents in Edgartown, Pennsylvania. Looking through a window, she sees Janelle crying, and, recognizing the toll that her cannibalism has taken on her mother, Maren decides to leave her alone.   

Alone in Edgartown, she is briefly taken in by the grandmotherly Mrs. Harmon, who offers to teach Maren how to knit but passes away in her sleep before she can do so. Maren lingers in Mrs. Harmon’s comfortable home until she wakes up to the sounds of Sully, an elderly “eater,” consuming Mrs. Harmon’s corpse. Shocked to discover that there are others who share her condition, Maren learns from Sully that there are many eaters in the world, mostly living on the periphery of society. Sully claims that he only eats the recently deceased, surprising Maren, who has no control over who she consumes. When Sully leaves, Maren continues her journey to find her father, who she believes might have also been an eater. Short on cash, she tries to hitch-hike to Minnesota, but is abandoned in a remote Walmart in Iowa. There, she consumes an employee named Andy and meets a handsome young man named Lee, who is also an eater. Lee offers to drive Maren to Minnesota after first driving down to Virginia to give driving lessons to his sister in fulfillment of a promise. Together, they travel down south, growing closer as they exchange stories from their lives and survive by sleeping outside or in abandoned homes.  

At a carnival, Maren and Lee encounter Sully, who invites them to spend the night at his cabin, though Lee is both distrustful of the older man and suspicious of the apparent coincidence of their reunion. The next morning, they leave Sully’s cabin and find Maren’s paternal grandmother, Barbara Yearly, who responds coldly to Maren. To Maren’s disappointment, Barbara informs her that Francis has been institutionalized in a mental care facility named Bridewell, and she gives Maren the facility’s address. Maren considers turning herself in to the authorities but is discouraged from making any rash decisions by Lee. That night, he tells her about his heartbreak following a break-up with a former girlfriend named Rachel, who witnessed him consume one of his mother’s violent boyfriends. Traumatized by what she observed, she was later institutionalized in a mental care facility. After Lee drops Maren off at Bridewell, Maren instructs him to go back to Virginia without her.  

At the asylum, she learns that her father has had no visitors in fourteen years and is in a profoundly poor state of mental and physical health. His journal, which is addressed to her, acknowledges that he is also an eater and that he was adopted by the Yearlys after killing and consuming a pedophile who abducted him. His journal details his profoundly unhappy childhood and his love for Janelle, who he meets and later marries while working as a forest ranger. Though he is at first able to hide his cannibalistic urges from Janelle, she grows afraid of him after witnessing him cannibalizing a man, and he leaves her while she is pregnant for her own safety and wellbeing. Maren flees Bridewell after the director calls Child Protective Services, and she seeks refuge in the home of Travis, an orderly at Bridewell who knows about eaters despite not being one himself. Maren is horrified when he requests that she eat him, and he apologetically offers to drive Maren to her next destination.  

He drives Maren to Sully’s cabin, where, to her shock and confusion, she finds her father’s forest ranger ID. After confronting him, Sully reveals that he is Francis’ biological father, and therefore, her grandfather. Their meetings have been no coincidence; Sully has been stalking Maren in order to kill and eat her. They fight, and Maren escapes after injuring him. She reunites with Lee, and after killing an unkind student at the University of Madison-Wisconsin, they live in her dorm room and enjoy the relative stability of campus life. One day, Sully arrives on campus and attempts, yet again, to kill Maren. However, she holds her own against him long enough for Lee to arrive and eat Sully. That night, Lee takes off his clothes and embraces Maren in bed. When she awakens, she realizes that he has eaten him, and she interprets his decision to embrace her as an expression of his consent to being eaten. Though she is sad, she continues to live on the college campus, working in the library and renting out a room in a local boarding house. Finally embracing her status as an eater, she intentionally seduces and presumably kills a young male student at the college.