Harriet M. Welsch (Michelle Trachtenberg) is probably the world's most accomplished 11-year-old spy. Harriet dreams of being a writer, and her nanny and best friend Golly (Rosie O'Donnell) told her to start by writing down everything she sees. It's all good fun until Harriet's friends find her secret spy notebook. They don't like what Harriet's written. And they don't like Harriet that much, either. Can Harriet win back her friends, or is she doomed to be an outsider, a rejected writer and forgotten spy?