Harriet M. Welsch (Michelle Trachtenberg) is an 11-year-old sixth grader and a young spy/writer who is best friends with Simon "Sport" Rocque (Gregory Smith) and Janie Gibbs (Vanessa Lee Chester). She lives a privileged life with her parents, Violetta and Ben and her nanny, Katherine "Ole Golly" (Rosie O'Donnell), who's the only person who knows all the things that Harriet has been snooping on. Harriet and her friends are enemies with a rich girl named Marion Hawthorne (Charlotte Sullivan). For a while, Harriet lives life very well with being a spy and having fun with Golly.
One night, being home alone with Harriet, Golly invites a friend, Mr. George Waldenstein, over. And after Golly accidentally burns their dinner, the three go out to dinner and a movie instead, where things turn into a disaster. Violetta fires Golly for letting Harriet stay out late, but then she realizes that she still needs her to look after Harriet and begs her to stay. Golly tells her, however, that she was planning to leave soon since she believes that Harriet is old enough to take care of herself, much to everyone's protests. Shortly before she leaves, Golly encourages Harriet to never give up on her love for observing people just because she'll no longer be with her, and promises her that she will be the first to buy her very own autographed copy of Harriet's first novel she sells in the future. After Harriet bids Golly goodbye, she becomes depressed and withdrawn. She even gets caught when investigating the home of Agatha K. Plummer (Eartha Kitt).
The next day, she plays with her friends at the park, and disaster strikes. Marion finds Harriet's private notebook and begins reading all of Harriet's vindictive comments on her friends out loud, such as how she suspects Janie "will grow up to be a nutcase", and mocking Sport's father for barely earning any money. Everyone finds that they're all cruel and hurtful, and even Sport and Janie turn their backs on Harriet. The kids then create a Spy-Catcher club and torment Harriet on her spy routes.
After running into a police officer, and then getting zeroes on her schoolwork, Harriet gets her notebook taken away by her parents. Her parents tell Harriet's teacher Miss Elson (Nancy Beatty) to search Harriet every day for notebooks, much to Harriet's embarrassment. One day, during art, Rachel Hennessey "accidentally" pours blue paint all over Harriet. The other students claim to help clean her up when they are really just smearing the paint around on her, and when Marion pours more paint on Harriet's hair, Harriet slaps her face with great force, leaving a blue hand print. As a result of this, Harriet does things to get back at everyone individually, then exposes to them that Marion's father left her because he never loved her. She then carries out some of her other plans, including cutting off a chunk of Laura's hair, messing up one of Janie's science experiments, which causes Janie's parents to punish her, and making a fake picture of Sport in a maid's outfit, severely humiliating the kids and strengthening their dislike for Harriet.
Harriet's parents find out what she has done to her classmates and send her to be evaluated by a psychologist, who assures them that Harriet is fine. Then things start to get better again. Harriet gets her notebook back, and she even gets a surprise visit from Golly, who tells her that in order to make things right again, she has to do two things: apologize and lie. When Harriet tells her that it's not worth it, Golly disagrees, and tells Harriet that she is worth it as an individual, and her being an individual will make others nervous (and keep making them feel as such), before finally adding that one of the blessings of life is good friends, and tells Harriet to never give up her friends without a fight.
Harriet then tries to apologize to Sport and Janie, even though they reject her at first (they later, however, get tired of being treated unfairly in Marion's bully group and quit). She also shares her opinion with Miss Elson about how getting appointed as the editor of the sixth grade paper wasn't being done in a fair way, and she get selected herself by her classmates, who get Marion's occupation as the editor voted out. Through one article, she apologizes to everyone, including Marion, and all (except Marion) accept her apology. All is well. On opening night of the 6th grade pageant, Janie, Sport and Harriet light off a stink bomb as revenge on Marion and dance to James Brown's "Get Up Offa That Thing" until the end of the film.Michelle Trachtenberg as Harriet M. Welsch
Rosie O'Donnell as Katherine "Ole Golly"
Gregory Smith as Simon "Sport" Rocque
Vanessa Lee Chester as Janie Gibbs
J. Smith-Cameron as Violetta Welsch
Robert Joy as Ben Welsch
Eartha Kitt as Agatha K. Plummer
Charlotte Sullivan as Marion Hawthorne
Teisha Kim as Rachel Hennessy
Cecilley Carroll as Beth Ellen Hansen
Dov Tiefenbach as Boy with Purple Socks
Nina Shock as Carrie Andrews
Connor Devitt as Pinky Whitehead
Alisha Morrison as Laura Peters
Nancy Beatty as Miss Elson
James Gilfillan as Archie Simmons
Gerry Quigley as Sport's Dad
Jackie Richardson as Janie's Mother
Roger Clown as Dr. Wagner
The film was released in US theaters on July 10, 1996, and the film grossed $6,601,651 on its opening weekend, averaging about $3,615 per each of the 1,826 screens it was shown on. The film went on to gross a total of $26,570,048 by November 10, 1996, and is considered a modest box office success, earning back more than double its $12 million budget.
Harriet the Spy was released onto VHS on February 25, 1997. The film was later released on DVD on May 27, 2003, with no special features whatsoever.
The film has received mixed reviews from critics and it currently has a 48% "rotten" rating on Rotten Tomatoes.
Another adaptation of Harriet the Spy was released as a television movie in 2010 entitled Harriet the Spy: Blog Wars, with Jennifer Stone in the title role.