Harriet the Spy has a secret notebook that she fills with utterly honest jottings about her parents, her classmates, and her neighbors. Every day on her spy route she "observes" and notes down anything of interest to her:

I BET THAT LADY WITH THE CROSS-EYE LOOKS IN THE MIRROR AND JUST FEELS TERRIBLE.

PINKY WHITEHEAD WILL NEVER CHANGE. DOES HIS MOTHER HATE HIM? IF I HAD HIM I'D HATE HIM.

IF MARION HAWTHORNE DOESN'T WATCH OUT SHE'S GOING TO GROW UP INTO A LADY HITLER.

But when Harriet's notebook is found by her schoolmates, their anger and retaliation and Harriet's unexpected responses explode in a hilarious way.