It was made into a 1996 film of the same name for Nickelodeon starring Michelle Trachtenberg. It was the first film made by Nickelodeon Movies. This was shot in the Florida cities Fort Lauderdale and Miami plus Toronto, Ontario, Canada.
Harriet's observations cover her family, caretaker, school, friends and spy route. After school, she goes to observe a set of people who have no idea of her existence. They include Harrison Withers, a bachelor with 26 cats; the Robinsons, a very wealthy but boring couple; Mrs. Agatha K. Plumber, an indolent divorcee; the Del Santis, an Italian immigrant family that runs a grocery store; and the Del Santis' deliveryman, Little Joe Curry, who has a habit of stealing food from the grocery for snacks and to give to a gang of hungry children who visit him regularly. Harriet considers herself a very good spy, having never been caught.
Harriet's best friends are Simon "Sport" Rocque and Janie Gibbs, and is friends with quiet Beth Ellen Hansen. (Sport and Beth Ellen eventually starred in their own novels by Fitzhugh.) Janie is interested in science, particularly chemistry, and is quite distrustful of her parents, especially her mother, who is trying to turn her into a "proper lady" by sending her to dancing school. Sport is a very mature boy who takes care of his father and himself by cooking, cleaning and managing the finances (Sport's mother abandoned him and his father and apparently has all the money). The reason this has happened is that his father, as an author, has no regular work and is often oblivious to his family's troubles. Beth Ellen is a beautiful, but very shy and timid girl from a rich family. Harriet finds Beth Ellen somewhat boring (and Janie outright despises her).
Among the other students in Harriet's class at school include rich, popular, class bully and perpetual teacher's pet Marion Hawthorne (described by Harriet in her notebook as a future "lady Hitler"); Marion's second-in-command, Rachel Hennessey; the repulsive Pinky Whitehead, whom Harriet describes as ugly; Laura Peters, who has a habit of smiling at everyone all the time; the somewhat pudgy Carrie Andrews, whose father is the Welsch's doctor; and a new student, the Boy With Purple Socks, who is so dull no one can remember his real name, which is Peter Matthews.
One evening, Ole Golly and her boyfriend, George Waldenstein, take Harriet out of the house to see a movie. At first, Ole Golly is reluctant to go, because she does not have Harriet's parents' permission; but George and Harriet persuade her that it is only a movie. Harriet adds that her parents are never home until late at night. After seeing the movie and stopping at the drugstore for sodas, the three come home to find that the Welsches have come home early. Mrs. Welsch is hysterical and fires Ole Golly on the spot; everyone is shocked, but Ole Golly stays calm. It is then revealed by Ole Golly that she would have left soon anyway, since she believes Harriet is at the age where she no longer needs a nurse and since George has proposed to her. To Harriet's displeasure and grief, Ole Golly leaves the next day. It is assumed that Ole Golly shortly thereafter marries her boyfriend and moves to Montreal, Québec.
One day during a game of tag, Harriet loses her notebook and is mortified when her friends find it and Janie proceeds to read all of Harriet's secret thoughts to everyone. The children find some of what she wrote hurtful, such as comparing Sport to "a little old woman" for his continual worrying about his father, or "Who does Janie Gibbs think she's kidding? Does she really think she could ever be a scientist?". Janie and Sport join the rest of the class in forming the "Spy Catcher Club", of which Marion Hawthorne declares herself president. The club meets regularly at Rachel Hennessey's house to think up ways to make Harriet's life miserable, including stealing her lunch (tomato sandwiches with mayonnaise), passing nasty notes about her in class, and having Rachel spill ink all over her and disguise it as an accident. Harriet regularly spies on them through a back fence. One day, out of utter frustration and envy, Harriet drops a note in the Hennesseys' mailbox for Rachel's mother. It states "All those kids hate Rachel. They just want your cake. Furthermore they will clutter up the backyard and also they constitute a nuisance. -A Friend" The Hennesseys find it and Rachel later announces to the Spy Catcher Club that a crank note was dropped in her mailbox. She summarizes what the note said, and to Harriet's amusement, Pinky Whitehead states "Well, it's very good cake".
The people on Harriet's spy route fare little better. The bachelor's cats are taken from him and he becomes depressed, the wealthy but boring couple receives a hideous sculpture to show off, the indolent woman reacts hysterically to her doctor's announcement that she must be confined to her bed for the rest of her life and the immigrant family's truck is ruined by their lazy son. And to her utter fury and humiliation, Harriet is caught in the dumbwaiter by Mrs. Plumber's maid.
Hurt and lonely, Harriet resorts to childish tantrums and resolves to get back at her former friends by thinking up a special punishment for each one. She gets into trouble when she carries out some of her plans (including cutting off a chunk of Laura's hair, hiding a frog in Marion's desk, and teasing Rachel about her estranged father). When that fails, Harriet tries to resume her friendship with Sport and Janie as if nothing ever happened, but they both reject her. On top of that, Harriet spends all her time in class writing in her notebook as a part of her plan to punish the Spy Catcher Club (in addition to "writing her memoirs"). As a result of never doing her schoolwork, her grades suffer, and Harriet's parents confiscate her notebook and take her to a psychologist. Exactly what the psychologist tells the Welsches is never found out, despite Harriet's assiduous spying; but before long Ole Golly writes to Harriet, telling her that if anyone ever reads her notebook, "you have to do two things, and you don't like either one of them. 1: You have to apologize. 2: You have to lie. Otherwise you are going to lose a friend."
Meanwhile, dissent is rippling through the Spy Catcher Club. Marion and Rachel are calling all the shots, and Sport and Janie eventually get tired of being bossed around by Marion and quit the club, inspiring most of their other classmates to do the same. Eventually all that is left of the club is Marion, Rachel, Carrie, and Laura, playing bridge and Mahjong in the afternoons, a caricature of the stereotypical suburban "ladies' club" of the 1950s. Harriet, spying on them, reflects with brief pity that they will probably do exactly that for the rest of their lives.
Harriet's parents speak with her teacher and the headmistress, and Harriet is appointed editor of the class newspaper (replacing Marion Hawthorne). The newspaper -featuring some stories about the people on Harriet's spy route, as well as juicy gossip about her schoolmates' parents (which Harriet has overheard from her own parents) - becomes an instant success. Things improve for those on her spy route as well: Harrison Withers obtains a new kitten, the Robinsons manage to find some people to look at their hideous sculpture, Mrs. Plumber, having received notice from her doctor that she really did not have to stay in bed, becomes full of bountiful activity, and the Dei Santis' "lazy" son becomes a very studious worker after he obtains a job he likes as a trucker.
After some time as the editor, Harriet makes amends to her former friends through the paper, offering a printed retraction and Janie and Sport forgive her in the end.