In the fourth and final book of this code-busting series, Marly, Sai, and Isla are back on the hunt. After their shocking discoveries on Summer Island, the Treasure Troop has one final set of clues to follow before reaching the end of their search. And now our trio must solve the toughest mystery of all: what really happened to Mr. Summerling?
 
Available in Hardcover, Paperback and Ebook

Penguin Workshop 10/12/21

HC ISBN: 978-0593094921, PB ISBN: 978-0593094914