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 (actor)) 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.