In 1883 Florida, Cassie Gwynne, a bookworm and cat lover, arrives in Fernandina harbor seeking a fresh start. She discovers the body of town grump Peanut Runkles while exploring the island with her aunt Flora. Flora, who had a long-standing feud with Peanut, is arrested for his murder. Determined to prove Flora’s innocence, Cassie embarks on an investigation, adopting a kitten named Esy along the way. She finds a tattered notebook in Peanut’s desk that may be linked to the crime. Cassie races against time to solve the mystery and save her aunt.