Can 12-year-old Thunder and her friends unravel a Revolutionary War-era spy mystery involving a hidden diary and buried treasure before the dangerous Fairmount Prowler does?
A suspenseful middle grade novel about history, found family, girl power, and critical thinking.
Is a hidden diary the key to a mysterious spy ring from the American Revolution?
Twelve-year-old Theodora “Thunder” Underwood hates summer vacation. Her parents go off to remote corners of the world on scientific expeditions and leave her with some relative or other who clearly doesn’t want her around. But maybe this year, 1979, will be different…
Thunder is spending her summer at Mount Pleasant, a crumbling old mansion outside Philadelphia once owned by Revolutionary War traitor Benedict Arnold. There, Thunder discovers a mysterious diary, written by Mercy, a girl her age who lived through the American Revolution. She’s plunged into a mystery involving a hidden ring, which she and her new friends must solve before the Fairmount Prowler—a notorious local burglar—does.
Book Details
Format:
Hardcover
Price:
25.99 CAD / 19.99 USD
Published: 07/14/2026
ISBN: 9781635923261
Imprint:
Calkins Creek
Page Count: 368
Trim Size: 5-1/2 x 8-1/4
Grades:
Grade 5 & Up
Ages:
10 and up
“A dazzling plot with characters who are rich and real and unforgettable.” —Lynda Mullaly Hunt, New York Times best-selling author of Fish in a Tree
“Barr’s Thunder and Mercy brilliantly draws young readers into the past through a clever and relatable main character whose voice feels fresh and alive. What makes it stand out is the way it balances that energy with meticulous attention to historical detail—small, surprising touches that deepen the story and bring its world vividly to life. It’s both engaging and richly grounded, a combination that will spark curiosity and open new doors to history for young audiences.”—Justina Barrett, chief learning and engagement officer, Historical Society of Pennsylvania, and former site manager for Mount Pleasant