The Spy Club explores Kelsey's new house, finding an abandoned tree house, a wad of cash, a missing heirloom, and a mysterious creature roaming the grounds.

Kelsey's dad has a new job and that means a new house—on the same property as a castle! Kelsey, Becca, and Leo can’t wait to explore, but what they find might lead to their most intricate case yet. How are an abandoned tree house, a wad of cash, a missing heirloom, and a mysterious creature roaming the grounds all connected? The Curious Cat Spy Club is determined to find out!

"Kelsey's dad has a new job and that means a new house--on the same property as a castle! And this castle has its very own mystery. A valuable chess piece has gone missing, and Kelsey is determined to unmask the thief. But there's more than one mystery on the property. When a shadowy creature roaming the grounds leads Kelsey, Becca, and Leo to a secret clubhouse in a tree, they must work together to find out who used to meet there. Could the elusive creature be the key to solving both mysteries?"--

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.

When Kelsey and her friends find an abandoned pug puppy, it looks like another case for the Curious Cat Spy Club. But shortly thereafter, Kelsey's mother goes missing—possibly in the line of duty as an Animal Control officer—and it's up to the CCSC to discover what happened. As they search for Kelsey's mom, they start to wonder if her disappearance is somehow linked to the puppy they found. The fifth book in the Curious Cat Spy Club series is filled with twists and turns and, of course, tons of animal adventures!

When a mysterious stranger claims to own Becca's beloved zorse, Zed, the Curious Cat Spy Club fears he might be responsible for abusing Zed in the past. Kelsey, Becca, and Leo are determined to uncover the truth before they have to give him away. But when a daring rescue attempt puts Kelsey in danger, does the CCSC have enough spy skills to save her, or are they in over their heads?