Aza addresses the reader and wonders if she's fictional. She sits in the school cafeteria eating and says that the human body is made up of about 50% bacteria. Aza has anxiety problems and can barely concentrate on the conversation around her: her best friend, Daisy, is talking with another friend, Mychal, about Mr. Pickett, who mysteriously disappeared after being accused of bribery and fraud. Aza wonders if she has C. diff, a serious bacterial infection, and she opens a crack in her finger and re-bandages it. After school, Aza and Daisy head to Applebee's to do homework. They hear on the radio that Pickett Engineering is offering $100,000 to anyone with information about Mr. Pickett's disappearance. Daisy reminds Aza that she knows Mr. Pickett's son, Davis, and the two decide to sneak onto the Pickett property to access a motion-activated camera that might have captured Mr. Pickett's departure. Daisy and Aza paddle down the White River in Aza's canoe. At the Pickett residence, Aza downloads a picture of Mr. Pickett from the camera before the security guard, Lyle, catches them. Daisy tells Lyle that their canoe has a hole in it, and that they know Davis. Lyle takes the girls to Davis, who remembers Aza. Davis introduces the girls to Malik, who cares for the tuatara, Tua, that lives on the property. Davis drives Aza and Daisy home. Later that night, Daisy shows Aza an article online that says Tua will inherit the Pickett estate when Mr. Pickett dies.
The next day, Aza drives around the circular freeway for a while before heading home. She meets Daisy at Applebee's after Daisy gets off work at Chuck E. Cheese's, and the two begin to look for clues about Mr. Pickett's disappearance. Aza finds Davis' social media channels, but he stopped posting after his dad disappeared. Daisy phishes a junior reporter and obtains the missing person report for Mr. Pickett. At school the next day, Aza reads the report and realizes that she and Daisy know more than the police since they have the picture from the motion-activated camera. Aza insists on talking to Davis before they go to the police. She and Davis text that night. The next morning, Daisy tells Aza that she agreed to go on a date with Mychal, but made it a double date with Aza and Davis. Aza invites Davis and he agrees to come. After school, Aza has an appointment with Dr. Singh, her psychologist. Aza wonders aloud if she's soulless and insists she's crazy, though she lies and says she's taking her medication regularly. As the session goes on, Aza's stomach hurts and she fears she has a C. diff infection.
On date night, Aza’s mom insists that Aza be careful with Davis because wealth makes people careless, but Aza brushes her off. At Applebee's, Aza floats in and out of the conversation, which is mostly about Star Wars and the fanfiction stories that Daisy writes. After dinner, Davis suggests they go to his house and watch a movie since he has a theater in his house. When the group gets to the Pickett mansion, Mychal and Daisy look at the art while Davis shows Aza the theater in the basement. Davis uses the sound system in the house to tell Mychal and Daisy that he and Aza are going outside and then leads Aza to the golf course. He points out stars and takes her hand. Aza admits that she hates living inside a body, and tells him that she opens the cut on her finger to prove to herself that she's real. The conversation turns to the subject of Davis' father. Davis insists that he can't trust Aza and Daisy to keep quiet. He leads Aza to a cottage, where he pulls $100,000 out of cereal boxes and gives it to her. Aza runs back to the house and interrupts Daisy and Mychal kissing. On her way out of the house, Davis' little brother, Noah, asks Aza if he can give her the notes from his dad's phone to help her search. He cries, and she suggests he go to bed.
Aza texts Daisy in the morning and they agree to meet at Applebee's. Aza researches items from the list of notes off of Mr. Pickett's phone, but is stumped by the phrase "the jogger's mouth." She gets to Applebee's early and goes through her dad's old phone. He liked taking pictures of the sky through branches. She remembers how he died: he just dropped dead while mowing the lawn. Daisy arrives and Aza shows her the money. Daisy nearly cries and believes she'll be able to go to college and quit working at Chuck E. Cheese's. They eat and order more expensive items than they usually do, and tip their server, Holly, very well. Davis's lawyer, Simon Morris, calls Aza the next day. He explains that he set up an appointment for her to deposit her money, and explains that Mr. Pickett won't be legally dead for another seven years if they don't find his body.
On the way to make the deposit with Daisy, Aza's mind begins to spiral. She changes her Band-Aid several times before Daisy finally encourages her to finish driving to the bank. After they deposit their money, Aza drops Daisy off. When she gets home, Aza takes a pill to help her deal with feelings of panic. Aza avoids her mom and accepts an invitation from Davis to watch a meteor shower on Thursday. On Thursday, Aza finds Daisy in the school parking lot with a new, orange VW Beetle. Aza thinks this is an irresponsible purchase, but Daisy hands Aza a guide to colleges and Aza spends the day engrossed in the guide. Aza and Daisy to go Aza's house after school, where Daisy pulls out a newly-purchased laptop. Aza is disapproving, but Daisy insists that Aza knows nothing about being poor. When Davis arrives to pick Aza up, Aza’s mom insists on talking to Davis. She tells him that his money doesn't entitle him to Aza, which makes him cry. Aza apologizes for her mom in the car. Davis and Aza pick at dinner and then go outside to look at the sky, even though it's cloudy and they can't see the meteor shower. Davis admits that he writes poetry, and Aza kisses him. They kiss until Aza begins to panic about Davis' bacteria inside her. She frantically checks her phone and finds out that Davis' bacteria will be inside her forever. Davis suggests they watch a movie and they both try to act normal. After Davis finally takes Aza home, Aza’s mom sings Aza a lullaby to help her fall asleep.
Aza and Davis decide to unofficially date. She has an appointment with Dr. Singh the next day, and she tells Dr. Singh about her panic attack while kissing Davis. She says that she fears she's a fictional story told by her body and her out-of-control thoughts. Dr. Singh insists that Aza is indeed real, but that she gives her thoughts too much power. Aza and Davis text later that night, and Aza enjoys it. On Monday, they see each other again at Davis' house. Davis says that Noah got suspended for bringing pot to school. They watch Jupiter Ascending and kiss in the theater, but Aza has to stop. She goes to the bathroom and drinks hand sanitizer, and Noah catches her on her way back downstairs. He asks why his dad isn't contacting him, and Aza feels her spiral tightening.
Later, Aza successfully discovers Davis' secret blog. The two facetime (that is, talk over a video call), and Aza feels close to Davis. They continue to facetime nightly for weeks. One night, Aza decides to read Daisy's fanfiction. One character, Ayala, is horrible: she's anxious and ruins everything. Aza realizes that the character of Ayala is modeled after her and that Daisy sees her as useless and helpless. Aza and Daisy watch a movie after school the next day, and the two argue a little before Daisy goes to hang out with Mychal. Later that night, Aza goes to see Davis. Davis says that he read Daisy's fanfiction and likes Ayala. He's spinning his Iron Man action figure and tells Aza about how the galaxy is a spiral. Aza tells Davis about her spirals, and then the two swim in the heated pool. Afterwards, Davis shows Aza his telescope. He tells her that he looks at the stars to remember his mom. When she gets home, Aza checks Davis' blog and then reads more of Daisy's fanfiction. She comes across a Wikipedia article about how gut bacteria communicate with the brain, which she finds horrifying. She drinks hand sanitizer and wonders if she's a threat to herself.
Aza’s mom wakes Aza up the next morning. Aza is sick and exhausted. She thinks she's never going to get better. On the way to Applebee's after school, Aza confronts Daisy about the fanfiction. Daisy angrily talks on and on about how Aza is exhausting, spoiled, and a bad friend. Aza turns to Daisy and yells that she can't escape from her mind. They rear-end the car in front of them, and two cars behind them collide into Aza's car. Aza is dizzy with pain, but knows she has to get her dad's phone out of her car's trunk. The phone is broken. A firefighter assures Aza she's okay and Aza blacks out. She wakes up in the hospital. The doctor finally tells Aza and her mom that Aza lacerated her liver and will need to stay in the hospital for several days. Aza panics and is afraid she'll get C. diff. She tries to distract herself, but her brain tells her she's going to die. It tells her to get up and drink hand sanitizer. Aza does, and Mom catches her as Aza starts vomiting. Aza realizes she's not possessed: she herself is the demon. When Aza wakes up the next morning, she tells her mom that she's in big trouble.
Aza offers an alternate ending in which she walks into the sunset with Davis or Daisy. In reality, Dr. Singh arrives and tells Aza she has to take her medication and that she will survive. She continues to check on Aza even after she goes home from the hospital. Aza doesn't accept visitors for the two weeks she spends in bed, and returns to school in December. Daisy meets her at the steps of the school and tells her everything that happened in the last two weeks. They eat lunch outside together, and Daisy tells Aza a story about a woman who insisted that the world rests on the back of a turtle, which rests on the back of another turtle: turtles all the way down. Aza thinks it perfectly describes her mental state. Later that night, she tells her mom about the money Davis gave her and agrees to meet Davis for dinner. At Applebee's, Aza panics again about Davis' bacteria and tells him they can't be in a relationship because this is as "better" as she's going to get.
The next day, Daisy invites Aza to go with her and Mychal to an art show in the sewer. Aza agrees. At the show, she and Daisy walk through the sewer and view the work. When Aza gets anxious, she asks Daisy to take a walk and explains to her that walking in a sewer with light isn't scary, but feeling constantly in the dark is. The two talk at the opening of the Pogue's Run tunnel until they realize they're in “the jogger's mouth.” They remember the stench and walk back to the gallery to get Mychal. At home that night, Aza tells her mom that she fears she discovered Mr. Pickett's body. Aza tells Davis the next day, and Davis cries. Davis doesn't text Aza again. Months later, Aza sees on TV that the police discovered Mr. Pickett's body in the Pogue's Run tunnel. She texts Davis, who tells her that he and Noah told the police. In April, Davis knocks on Aza's door. He gives her a gift—it’s the spiral painting by Pettibon—and tells her that he's moving to Colorado. Aza then addresses the reader directly, explaining that she didn't know at that time that she would go on to grow up and have children, though she'd get too sick to care for them twice. She wrote this story when a psychiatrist asked her to write how she got where she is now. She learned through writing that she would always go on as a singular being.