The book begins with Susannah relating a hallucination from the hospital. She wakes up to find that she's restrained. A “purple lady” tries to soothe Susannah, but Susannah becomes angry and paranoid. She begins to pull wires off of her head and notices an orange band on her wrist that says, "FLIGHT RISK."
Susannah then goes back to when her illness began. She wakes one morning to find bites on her arm, which she attributes to bedbugs. The next day at work, she tries to talk to her friend Angela about her bites, but is interrupted when her phone rings. Susannah is horrified when she hears Steve, the Sunday editor, on the phone—she forgot about her weekly meeting to pitch stories. Later, Susannah throws away all her saved articles that she wrote to prepare for the exterminator. Several days later, Susannah is suddenly gripped with overwhelming jealousy and decides to read her boyfriend Stephen's emails. She spends two hours going through emails and his dresser, but thinks that something is wrong with her if she's doing this. Then her left hand goes numb, which she finds just as distressing as her paranoia. A coworker suggests that Susannah see a doctor. When she calls her gynecologist, Dr. Rothstein, he refers her to Dr. Bailey, an esteemed neurologist. Dr. Bailey's neurological exam yields normal results, but he orders blood work and an MRI. At the lab, a male technician guides Susannah through the MRI process. She feels he's too flirtatious and leaves quickly, forgetting her lucky ring in the process. A few days later, Dr. Rothstein calls Susannah and says she likely has mono.
When Susannah is feeling better a few days later, Stephen takes her to a concert. Susannah cannot eat any of her food, is nauseous and dizzy at the concert, and admits afterwards that she can't even remember it. When Susannah finally informs her parents that she's feeling sick, her parents are worried. A few days later, Susannah is feeling better and convinces Stephen to go with her to Vermont. When she tries to ski, she has a panic attack at the top of the mountain.
A week later, Steve asks Susannah to interview John Walsh, the host of the show America's Most Wanted. On the day of the interview, Susannah walks down the hallway at the New York Post and notices that the walls are breathing and the ceiling is expanding. Susannah can barely follow Mr. Walsh as she interviews him, and doesn't remember walking home that night. The following morning, Susannah notices that the billboards in Times Square are bright and vibrating. She wants to vomit and barely makes it to work. When she finally arrives, Susannah experiences mood swings at her desk—she goes from angry and frustrated to intense happiness in the span of minutes, and sobs through it all. That night, Susannah cannot fathom eating any of Stephen's carefully prepared dinner. When he turns on the TV and falls asleep, Susannah has a seizure.
Susannah regains consciousness in the ER and believes she's dying of melanoma, which she had several years ago. The ER doctor insists that they cannot keep Susannah, and Stephen calls Susannah's mom. The next day, Susannah's mom and her husband, Allen, arrive in Manhattan to convince Susannah to move home to Summit, New Jersey. Susannah experiences more seizures there, and her mom takes her back to see Dr. Bailey. He prescribes Susannah anti-seizure medication and refers to her a psychiatrist. The psychiatrist prescribes anti-psychotic medications. A few days later, Susannah has another horrifying seizure. Susannah's mom makes an emergency appointment with Dr. Bailey, where a nurse tests Susannah's brain's electrical activity with an EEG test. Though Susannah is obviously unwell—she attempted to jump out of a moving car on the way to the appointment—the results are normal. Susannah accuses her mom of trying to trick her.
Susannah's mom arranges for Susannah to spend the night with her dad in Brooklyn. Her dad and his wife, Giselle, accompany Susannah to her apartment before they go to their house. Susannah accuses her dad of kidnapping her. When they finally reach Brooklyn, Susannah is exhausted and hears Giselle call her a spoiled brat. She spends the night vacillating between asking Dad to sit with her and banishing him from the room. She tells him horrible things and hallucinates that he's killing Giselle. In the morning, Susannah's parents speak and decide that Susannah needs to be admitted to the hospital. They take her back to Dr. Bailey, who insists that she's suffering from alcohol withdrawal but finally arranges a bed for her at NYU. In the lobby, Susannah leaves to buy a coffee. She suffers a seizure and doesn't remember any of the next month.
The hospital staff connects Susannah to a 24-hour EEG machine. Her room on the epilepsy floor also has cameras to record seizures. She insists that her dad and Allen not be allowed into her room, and hallucinates that the nurses watch her use the bathroom. She panics and tries to escape. Dr. Russo and Dr. Siegel examine Susannah in the next day, and Susannah's mom takes an instant liking to Dr. Siegel. Over the next few days, more doctors join Susannah's team and suggest diagnoses such as unspecified mood disorders, unspecified psychotic disorder, or bipolar disorder. She continues to hallucinate that people on TV are talking about her, which leads to another escape attempt. When the nurses imply to her dad that she'll be moved to a psych ward if the escape attempts continue, her dad begins reading outside her room every day.
After a week, Susannah's psychosis recedes, and the hospital staff schedules a spinal tap. As the second week begins, Susannah begins slurring her words and allows her tongue to hang out of her mouth. Dr. Siegel explains to her parents that the spinal tap shows elevated white blood cell levels, which indicates an infection. Several friends come to visit Susannah over the next few days and are shocked by her appearance, and Susannah's mom is similarly shocked when she learns that Dr. Siegel is no longer working on Susannah's case. Dr. Russo explains that a doctor named Dr. Najjar has taken over and would like another spinal tap. The test reveals white blood cell levels that are much higher than the first test, which indicates that Susannah has a form of encephalitis (brain swelling). A few days later, Dr. Najjar finally arrives to introduce himself to Susannah. He asks her to draw a clock face from memory. Painstakingly, Susannah draws a clock with all the numbers on the right side, which indicates that the right side of her brain is inflamed. Dr. Najjar arranges to perform a brain biopsy and sends samples of Susannah's blood and cerebrospinal fluid to Dr. Dalmau, who discovered anti-NMDA receptor autoimmune encephalitis in 2007.
The hospital conducts tests to measure Susannah's cognitive function and ability to speak, and she scores poorly on all of them. She's diagnosed officially with anti-NMDA receptor autoimmune encephalitis that afternoon and is informed that she'll need to undergo an ultrasound to check her ovaries for teratomas, a type of tumor. Susannah hallucinates during the ultrasound, which reveals no tumors. Dr. Najjar explains his treatment plan to Susannah and her parents, which he believes should return to Susannah to 90% of her former self. Susannah's parents and a friend from college help move Susannah home the next day. Susannah becomes paranoid that Stephen will leave her for her friend, but this is part of the normal recovery process—patients go through the same symptoms in reverse as they recover.
Susannah says that it's impossible to talk about what it was like to be crazy, as she doesn't remember it and didn't have any self-awareness during that time. A few weeks after Susannah's release, one of Stephen's nephews is scared of her, and she begins to understand that she's different than she used to be. As Susannah attends family weddings and parties over the summer, she struggles with shame about her appearance (the steroids make her face puffy and caused her to gain weight) and the fact that she has a hard time holding conversations. People speak slowly to her because they don't realize she understands them perfectly—she just can't speak well. She has to take medications six times per day and resents her mom when she hounds her to take them. Her mom also insists that Susannah be evaluated at a rehab center. Though Susannah scores abysmally on some tests, she aces those that measure her analytical thinking. This indicates that Susannah is fighting her body; her mind is as sharp as ever.
During her third hospital stay for treatment, Susannah begins keeping a diary and becomes curious about what happened to her. Her dad helps her with her journal and is alarmed to discover that Susannah remembers nothing after her seizure in the hospital lobby. He finally gives her his personal journal from her time in the hospital because he finds the experience too difficult to talk about. Susannah's relationship with her mom improves when her mom finally admits how she scared she was that Susannah would die.
Though Susannah continues to improve, she moves in permanently with her mom and Allen. For her, this represents giving up her freedom. To combat this, Susannah begins keeping lists and reading. She also begins exercising to lose weight, though she admits that her fixation on her body only covered up her fears that she'd never be the same again.
In the fall, Susannah finally returns to work. She writes several articles from home before returning in full capacity. After attending one of Dr. Najjar's lectures on Susannah's disease, Steve asks Susannah to write an article about her experience. As she researches, she's shocked to discover that though the disease was only recently discovered, doctors believe it's been around forever—and is likely to blame for the symptoms that prompted exorcisms, especially in children. She also contacts Dr. Bailey and discovers that he'd never heard of the disease. After the article runs, Susannah's inbox floods with emails from people with the disease themselves, or with afflicted family members.
Susannah admits that she only remembers her hallucinations from the hospital, which doctors attribute to the fact that hallucinations are created by the brain and are therefore flagged as more important. As Susannah continues to recover, however, she finds that some things trigger faint memories of her time in the hospital. Two years after her release, Susannah returns to the hospital. She runs into the “purple lady” at the nurse's station, and the nurse embraces her. Though Susannah's opening hallucination wasn't real, the nurse was. A year later, Susannah visits a patient at NYU who had been referred by Dr. Bailey.