Kelsey can't resist collecting secrets in her spy notebook—just like her hero, Harriet the Spy. When she learns Leo has been hiding something from the group, she writes his secret in her notebook as well. But Kelsey accidentally brings her Notebook of Secrets to school and loses it. Everything she's collected about classmates, friends, and family could be released into the world!
When she receives a ransom note, she tries to solve the mystery on her own but soon realizes she needs everyone in the Curious Cat Spy Club to rescue the notebook, help a homesick 130-year-old Aldabra tortoise, and unmask a thief. A surprise ending changes Kelsey's life in a way she never imagined.