The book begins with thirty-year-old Margaret Ingraham deciding that her coworker Patrick McMahon needs to die. Margaret is an executive assistant at the marketing firm Kline, Martin, and Frankel in Silver Lake, Arkansas, and she lives an extraordinarily controlled life shaped by her late mother, an agoraphobic and obsessive woman who taught Margaret to fear germs, disorder, strangers, accidents, and almost every form of independence. Margaret checks locks and stove burners repeatedly, counts things, dislikes odd numbers, cleans obsessively, and has spent most of her life making herself inconspicuous. Her mother died only months earlier, leaving Margaret alone in the family house and beginning a slow, confusing process in which she starts discovering what she actually wants. Her main source of affection is June Ellen, the unruly seven-year-old girl next door. June Ellen’s mother, Louisa, is messy, distracted, and inconsistent, so Margaret often takes the child to the library, feeds her, bakes with her, and gradually loves her almost like a daughter. At work, however, Margaret is tormented by Patrick. He deliberately calls her Peggy, Pegs, Mouse, and other names she hates, steals food and supplies from her desk, makes sexual comments, humiliates her, and constantly promotes artificial-intelligence software that he claims could replace administrative assistants. Because Margaret sees her job as one of the few stable achievements of her life, Patrick’s campaign feels like an existential threat. Her first solution is murder. She remembers that Patrick claims to have a deadly peanut allergy, steals the EpiPen from his desk, and bakes peanut-butter muffins disguised with banana and coconut. She leaves them where she expects Patrick to steal them. He does, but nothing happens. Margaret waits for him to go into anaphylactic shock and finally realizes that his supposedly deadly allergy is another lie. She is simultaneously relieved that she has not killed him and furious that he fabricated the allergy, apparently forcing the entire office to give up foods containing nuts simply because he enjoyed controlling everyone.
Margaret begins looking for another way to remove Patrick without murdering him. At the same time, the rest of her carefully ordered existence begins changing. A previously unknown cousin, Jimmy Olmstead, claims that his father was Margaret’s estranged uncle and sues her for a share of the family property, potentially costing Margaret the house she has lived in all her life. Instead of accepting arbitration, she tells her attorney, Gregory Charles, that she wants to fight. She cuts off the long hair her mother always insisted she keep, gets highlights, starts wearing lipstick, buys a smartphone, gets internet service, drives her mother’s old Crown Victoria again, and begins going to the office’s “Thirsty Thursday” gatherings. Her first night drinking ends with too much tequila and vomiting, but something far more important happens beforehand: she sees Patrick secretly drop something into a young woman’s drink. Margaret deliberately knocks the drink over before the woman can consume it. From that point on, she becomes convinced that Patrick is not merely a workplace bully but a sexual predator who drugs women. She starts trying to obtain video evidence, even following him to a nightclub while disguised in clothes and makeup borrowed from Louisa. Patrick notices her, becomes suspicious, and turns their conflict into an increasingly personal contest. Meanwhile the book introduces a second criminal thread. FBI agent Joy Flash comes to the office because Patrick stayed at a Mississippi hotel near the place where a woman named Carla Melton disappeared and was later murdered. Silver Lake police are also reopening a series of unsolved murders attributed to a possible serial killer nicknamed the Silver Lake Strangler. Lieutenant Tom Emerson runs the cold-case investigation. Margaret has already seen him on television and later meets him by chance in an ice-cream shop with June Ellen. She begins to think Patrick may be connected not only to drugging women but to the stranglings.
Patrick retaliates against Margaret in increasingly serious ways. Important files disappear, including backup copies Margaret keeps for her incompetent boss, making her look unreliable precisely when the firm is discussing layoffs. Patrick hints that he knows Margaret took his EpiPen. Margaret becomes desperate to obtain undeniable proof against him. She steals Patrick’s keys long enough to make impressions, manufactures crude duplicate keys, and eventually breaks into his house while he is supposed to be out drinking. The copied keys fail, so she climbs through an unlocked window instead. She searches his kitchen, bathroom and bedrooms for drugs or trophies and finds nothing conclusive. Out of spite she deliberately clogs his toilet, hoping it will overflow and damage his floors. Then Patrick unexpectedly returns with a severely impaired young woman. Margaret hides in the closet of the guest bedroom. From the narrow gap between the doors she watches a dark-haired man bring the woman into the room, restrain her ankles with belts, place her on a plastic shower curtain, hit her, tape her mouth, and put a rope around her neck while Carpenters music plays repeatedly. Margaret assumes the attacker is Patrick because she is inside Patrick’s house and because everything she already believes about him seems to fit. When the sabotaged toilet begins flooding, the attacker leaves the bedroom. Margaret barricades the door, frees the drugged woman, removes the rope and duct tape, pushes her through a window, and escapes after her. She sees the naked victim apparently flag down a passing car and believes the woman is safe. Margaret herself runs back to the Crown Victoria but discovers later that she has left her grandfather’s monogrammed screwdriver behind, potentially connecting her to the illegal break-in.
