Eleven-year-old Harriet M. Welsch is an aspiring writer who lives in New York City's Upper East Side. Encouraged by her nanny, "Ole Golly", Harriet observes others and writes her thoughts down in a notebook as practice for her future career. Several standalone episodes highlight the various eccentric characters she meets on her afternoon "spy route".
Harriet's best friends are Simon "Sport" Rocque, a serious boy who wants to be a certified public accountant or a ball player, and Janie Gibbs, who wants to be a scientist. Harriet's enemies in her class are Marion Hawthorne, the teacher's pet and self-appointed queen bee, and Marion's best friend, Rachel Hennessy.
Harriet's life changes abruptly when Ole Golly's suitor, Mr. Waldenstein, proposes and she accepts. Harriet is crushed by the loss of her nanny. Her mother and father are at a loss to understand Harriet's feelings and are of little comfort to her.
At school, during a game of tag, Harriet loses her notebook. Her classmates find it and are appalled at her brutally honest documentation of her opinions of them. The students form a "Spy Catcher Club" in which they think up ways to make Harriet's life miserable, such as stealing her lunch and passing nasty notes about her in class. In return, Harriet regularly spies on the Spy Catcher Club through a back fence and concocts vengeful ways to punish them. After getting into trouble for carrying out some of her plans, Harriet unsuccessfully tries to resume her friendships with Sport and Janie as if nothing had ever happened. When Harriet's grades go down, her parents confiscate her notebook, which only depresses her further.
Harriet's mother takes her daughter to see a child psychiatrist, who advises Harriet's parents to get in touch with Ole Golly and ask her to write to Harriet. In her letter, Ole Golly tells Harriet that if anyone ever reads her notebook, "you have to do two things, and you are not going like either one of them. 1: You have to apologize. 2: You have to lie. Otherwise you are going to lose a friend."
Meanwhile, dissension is rippling through the Spy Catcher Club. Marion and Rachel are calling all the shots, and Sport and Janie are tired of being bossed around. When they quit the club, most of their classmates do the same.
Harriet's parents speak with her teacher and the headmistress, and Harriet is appointed editor of the class newspaper, replacing Marion. The newspaper—featuring stories about the people on Harriet's spy route and the students' parents—becomes an instant success. Harriet also uses the paper to print a retraction of the things she had written in her journal. Harriet is forgiven by Sport and Janie.
