Harriet the Spy is the story of a girl who observes those around her and keeps notes on what she sees. She actively spies on her family, classmates, and people in her neighbourhood, noting such failures as indolence, snobbishness, and generational conflicts. One person she spies on, an artisan named Harrison Withers who makes intricate hand-made birdhouses, keeps 26 cats. Her blunt appraisals of everyone around her are accurate but uncharitable: even her closest friends are critically appraised. Her best friend, Sport, is the son of a single father who, as a novelist, can barely keep food on the table. Sport has taken on the role of housekeeper for his father. Harriet’s other close friend, Janie, is a budding scientist, more interested in experiments than in typical childhood pursuits. All these quirks are duly noted by Harriet. She comments also on her own quirks, for example, her habit of eating nothing but tomato sandwiches for lunch.
The book is set on the Upper East Side of Manhattan among a mostly upper crust cast of characters. Harriet’s father is a television producer, and her mother does not work outside the home. She is cared for by a nanny, known to her as Ole Golly (her parents call her “Miss Golly”). The family has a cook. Harriet and her friends attend an elite private school (Sport presumably as a scholarship student) called The Gregory School. The school offers such amenities as a school newspaper, of which Harriet longs to be 6th grade editor; and a drama program which involves an idiotic Christmas pageant in which the 6th grade represents Christmas dinner, and Harriet is assigned the role of an onion.
Harriet’s individuality is recognized, however, by Ole Golly, who encourages her to write and who broadens her knowledge of the world and literature. When Ole Golly leaves to get married, Harriet is thrown back on her own ingenuity. Thwarted in her efforts to edit the newspaper, still practicing her onion role, and finally caught spying in the dumbwaiter of a big house, she is unable to settle into her usual routine. One day, rather than go out on her spy route after school, she plays with the other students in her class. Fatefully, they pick up her notebook and begin to read the devastating items she has noted about each of them. Because they are so truthful, and so callously written, these notes have a real sting to them. She is ostracized by everyone at school, taunted and bullied. She attempts to get back at them, thinking up different punishments that will be most painful to each person.
Her parents finally realize what is happening to her, and begin to take action to help her. At first locating the notebook as the source of the problem, they take it away from her; later, realizing this has only exacerbated the problem, they take her to a psychiatrist who realizes that Harriet requires her notebook to help her make sense of the world and to create meaning in her life. In effect, he recognizes Harriet as a writer. In a stroke of cunning, Harriet’s parents contact Ole Golly, who writes Harriet a letter of advice. Acknowledging that Harriet’s notebooks are for herself and nobody else has the right to read them, Ole Golly counsels that if anybody should happen to read her notebook, she has to apologize; and she has to lie. “Otherwise you are going to lose a friend.” Golly explains that white lies to make someone feel better are acceptable lies, but cautions, “to yourself you must always tell the truth.” In the same letter, Golly urges Harriet to take her notes and make them into a story, which she promptly does.
Finally, Harriet’s parents arrange for Harriet to take over editing the school newspaper. In the newspaper Harriet is able to use the fascinating details of people’s personal lives that she has found out through her spy route to write stories that everyone in the school wants to read. She also uses the newspaper to make amends, printing a “retraction” to the things she had written in her notebook. She is forgiven, and the book ends with Harriet working on her writing, keeping her notebook, and meeting up with her friends.