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 when the notebook goes missing, everything she's collected about classmates, friends, and family could be revealed to the world! After receiving a ransom note, Kelsey tries to solve the mystery on her own. But she soon realizes she needs help from everyone in the CCSC to rescue the notebook, help a homesick 130-year-old Aldabra tortoise, and unmask a thief.