Thunder and Mercy: A Novel

by Jennifer Robin Barr

Is a hidden diary the key to an unsolved secret from the American Revolution?

It’s 1979 and twelve-year-old Theodora “Thunder” Underwood is dreading her summer vacation once again. Every year, her scientist parents disappear on far-flung scientific expeditions, leaving her with relatives who clearly don’t want her around.

But this summer, Thunder is sent to Mount Pleasant, her aunt’s historic yet crumbling old mansion outside of Philadelphia once owned by Revolutionary War traitor Benedict Arnold. When Thunder discovers a mysterious diary written by Mercy, a girl her age who lived through the American Revolution, Thunder is plunged into a mystery that involves a shadowy prowler, a hidden ring, and mysterious spies—making her summer far more exciting than she ever expected.

Fast-paced and full of twists, this suspenseful middle grade novel transports young readers between the 1770s and 1970s as Thunder uncovers the truth behind a Revolutionary-era spy network—and discovers the importance of friendship, family, and her own voice along the way.

Genres: Historical Fiction, Middle Grade, Mystery, Historical, Family, American Revolutionary War, Fiction

368 pages, Hardcover

Published July 14, 2026

Reviews

[Amina]

✰ 3.5 stars ✰

“All anyone really needs is a new piece of evidence to change what we know about history.”

Thunder and Mercy was a bit on the longish side for me when it comes to middle grade reads' lengths. 😮‍💨 Maybe it's a sign of old age, but I felt it was more drawn out than it should've been. Not to mention, the constant alternation between 1979 events and Mercy's journal entries became taxing after a while.

The blend of history with mystery, secret codings, and puzzling forgery tied to the Culper Spy Ring and the build-up toward Benedict Arnold's infamous treason was done well. 📜 The author richly incorporated Philadelphia's historical landmarks into Mercy's past while creating present-day struggles that Thunder had to overcome.

“History can be really one-sided when it’s only been told by one perspective.”

Some of the current timeline issues were the weaker points of the narrative, such as the neighborhood burglar, which ultimately ended in a very Scooby-Doo-esque way that made me inwardly groan. 🙅🏻‍♀️ But, for younger kids, I suppose it serves as a lighter balance to some of the heavier and serious tones.

I liked how the author connected Theodora Underwood, aka Thunder, with Mercy's Revolutionary War struggles and her own experiences of feeling misplaced and lost. 🧭 It was a convincing likeness that gave her a stronger conviction and purpose to find out what really happened to her amidst the tumultuous time - even if telling a story through diary excerpts can only do so much.

It was also nice to see Thunder grow to appreciate her entomologists-parents' research ways in a new light. 🔎 How history and science share similarities, realizing that parts of their talent, skills, and curiosity in searching for clues made her feel closer to them, despite spending so much time apart most of her life.

“Bricks and mortar make a house, but loved ones make a home.”

She did act a bit abrasively, selfishly, if not inconsiderately at times; I know it's not fair to judge so harshly considering the rejection she's faced growing up and her parents' constant absence. 😕 But seeing how she hurt her new friends from her behavior was a cathartic and believable way for her to experience empathy and growth in others' kindness and friendship.

After some ups and downs, 8-year-old aspiring weatherman Charlie and his older sister, Heather, wormed their way into Thunder's heart. Eccentric and free-spirited Aunt Hazel was a breath of fresh air; I loved her pearls of wisdom, her graciousness, and her strength and fortitude in helping Thunder find herself in Mount Pleasant, a place to finally call home. 🫂

[Rachel]

Gr 4 & Up – Theodora Underwood, better known as “Thunder,” is used to being shuffled around while her research-scientist parents chase their next expedition. This summer she lands in Philadelphia at her eccentric Aunt Hazel’s home, Mount Pleasant, once owned by Benedict Arnold. It does not take long for Thunder to realize she has stepped straight into a layered mystery: a burglar is hunting for a historic ring, she uncovers a letter from 1780, and she discovers the diary of a girl named Mercy with ties to the Culper Spy Ring.
Set in 1979, the novel captures the freewheeling feel of a Philadelphia summer while weaving in high-stakes Revolutionary War espionage. Thunder is prickly but compelling, determined to prove herself apart from her science-obsessed parents by solving a centuries-old puzzle. Charlie, an earnest eight-year-old with meteorological aspirations and a permanent tie, and his sweet but struggling sister Heather round out this trio of latchkey sleuths who will inspire readers to form puzzle-solving crews of their own.
Barr moves confidently between eras, blending historical fiction with brisk, thriller-like momentum. The result is an immersive adventure filled with coded messages, spy lore, and the ache of growing up with largely absent parents. Informative back matter provides context on the real Culper Spy Ring and Benedict Arnold.
VERDICT: A must-purchase for middle grade collections. A lively, genre-blending mystery that will appeal to readers who enjoy historical intrigue, clever puzzles, and a strong-willed heroine.

[Kathleen Osterhoudt]

Fireworks! I read Thunder and Mercy on July 4, 2026 as the United States celebrated its Semiquincentennial. I was looking forward to reading a book set in Philadelphia with a Revolutionary twist. I was "Mount Pleasantly" surprised at what a propulsive read it turned out to be. As a local Philadelphian who grew up in the 70's I would say Barr nailed the setting and the story was very intriguing. If you like history, read it. If you like short chapters that keep you wanting more, read it. If you like thrilling dual timelines, read it. If you like mysteries, read Thunder and Mercy!

[Lisa Guzman]

This is a fast-paced middle grade novel that uncovers some of the history of the people surrounding Benedict Arnold and his betrayal. The main character is a girl named Thunder, who finds a mysterious journal written by a girl called Mercy back during the Revolutionary War. Thunder is curious and loves history, but she is also a lonely girl who struggles with making friends because her parents live unconventional lives (they are scientists, so they move around to study in the field and leave Thunder to her own devices.) Thunder is sent to live with her Aunt just outside of Philadelphia, in the mansion that Arnold bought for his wife. Her curiosity leads to her finding the journal and with her new friends, they set out to discover who was involved in the spy networks. The stuff about Thunder figuring out how to be friends with Heather and Charlie and the things with her parents being gone and her wanting to establish a home for herself were sweet and really made the book feel like a middle grade story. The history events were interesting.

[Janet]

Yes, this is a middle grade historical fiction novel and I personally would recommend it for fifth through eighth grade.
It has a double themes which are very well woven into one storyline and as an adult I enjoyed it. The setting is Philadelphia in the 1970’s interwoven with a mystery from the 1770s.
Thunder has just finished the school year and her professor parents are off on their annual summer long research project and Thunder always stays with some relative or connection during the summer. This year she is in Philadelphia with Aunt Hazel who lives in a less than luxury historic home Mount Pleasant that has plenty of intrigue and the mystery begins. Thunder is assisted in solving this historic mystery which is actually based on actual occurrences from Revolutionary War times by sibling Heather and Charlie.
Story moves along well and author includes six pages of info about the real historical people in this title who I knew little about. Now I am going to read further about Peggy Shippen Arnold.

[Rebecca]

3.5 Stars

I was really excited about this book, but it didn’t quite meet my expectations. It had a lot of good in it. I loved stories of the American Revolution and our main character was one to love. But I felt like the story almost did too much. There was a whole lot going on with finding a journal from the past to dealing with bullies at school to absent parents. I also felt like kids really needed a bit of background knowledge of the American Revolution to make much sense of the book. And it was really long, and I’m afraid it lost my interest at times. Some great themes and wonderful social justice moments included, but I just didn’t love this middle grades book.

[Kate]

i love my country Philadelphia!!! great middle grade historical fiction mystery set at Mount Pleasant!!!!!!

[Mia]

I devoured this middle grade book in two days! My fifth grade self would have absolutely loved to learn about the Revolutionary War through the adventure that is Thunder & Mercy.

Barr does a fantastic job of unraveling the history of the Revolutionary War through an exciting treasure hunt with many twists and turns. This book reminded me of one of my favorite movies (National Treasure) and I learned so many new and exciting facts about the war, Philadelphia and the Quaker tradition in this story.

Barr’s writing is informative while still creating engaging characters and story development. The representation of a female, black professor trying to make a name for herself in 1970s academia is a story not often told, and an important one. In addition to the adventure the underlying story of finding belonging and community is extremely relatable and relevant in today’s society.

I highly recommend this book for any history buff and/or those that love a touching, twisty adventure! 5 ⭐️s!

[Katie Hutchison Irion]

I enjoyed this historical fiction/mystery.

[Olivia Barr]

A must read novel that appeals to all! The author encapsulates the audience with mystery, while developing relatable and complex characters. Thunder’s character arc, specifically in her interactions with friends and family, is heartfelt and engaging. The nuggets of Philadelphia pride and historical references add another level of depth, making the story an absolute page turner. I highly recommend this book - my top read of the summer!

[Jennybeast]

I'm impressed at the intricacy of plot and history in this book. It's fast paced in some ways, but has the added counterpoint of '70s kids whiling away the hot and humid summer days in Philadelphia. Both Thunder and Mercy's voices are so strong from the beginning -- independent, lonely, trying fiercely to protect themselves from hurt and danger. I'm so glad that part of the story is about finding safe harbor and learning to trust again, and about how hard friendship can be. I love the intertwined story about the Culper Spy Ring and its impact on the American Revolution -- I am just as riveted as Thunder and her friends to be learning about this aspect of our history. It's a great story, with a lot to immerse yourself in and think about. Absorbing.

[Christine Doron]

Strength & Friends

Engaging story of a bright and curious almost 12 year old girl seemingly pushed off on relatives by parents caught up in their careers. More than she thought has rubbed off on her from her researching scientist parents. Thunder is going to make a bigger discovery than her globetrotting parents.

[Cynthia]

I’m not sure I could recommend this book more highly. I enjoyed it from page one and was heartbroken when it ended. I never wanted to leave this magical story.

[Lisa Lawrence]

An excellent YA book that deals with Benedict Arnold’s shift from a major general under Washington to a traitor. The story occurs at a real historic home in Philadelphia and several women who spied on the British army on Washington’s behalf.

[Angi]

I have been recently reading, middle grade books to sort of get away from a lot of the books written for adults that feel hopeless. This book does not feel that way if anything it gives me a lot of hope, and it encourages courage. Especially in our current time in the United States I feel like this is actually a perfect book to remember why we want to have and why we need democracy.

[Holly Wagner]

Meet Thunder—a force to be reckoned with. Her eleven year old self is independent, curious, and determined. Not to be undone by a mystery, a night prowler, mean girls, or a hurricane. Despite her absent parents, she discovers her Aunt Hazel at Mount Pleasant where she quickly becomes obsessed with finding a 200 year old journal of another young girl embroiled in the Benedict Arnold betrayal and the Culpepper Spy Ring.

Couldn’t put it down.

[Erica]

I could see a kid interested in history getting sucked into this boom!

[Anthony Rosato]

What a great, fun read! Amazing storytelling—this is the epitome of a page-turner, with characters you'll be rooting for from page one. The Philadelphia locations add a ton of flavor and immerse you in the story, but it's the storytelling that really is the star here. I really loved this book.

[Kimberly]

A fantastic middle grade historical fiction. I loved Mercy’s story and appreciated the authors note at the end explaining the what is known of the spy ring.

I loved Thunder and her passion for history. I loved her coming of age story and finding friends.


I didn’t understand her parents at all. Willing to essentially give up their daughter? Also, classic middle grade dilemma of “if you just TALKED to someone…”

[Karin Smith]

4.5

[Sara Hudson]

A well-plotted historical fiction mystery based on the spy ring that helped bring down Benedict Arnold. Set in Philadelphia in 1979, Barr gets the details of that time period just right - down to the Welcome Back, Kotter lunchbox and the friendship barrettes.

A word to the wise - the audio is a bit plodding. The narrator does a good job with the voices, but the pace is slow. Might benefit from 1.25 speed.

[Chelsie Jensen]

Middle grade historical fiction

[Lisa]

Rtc

[Ms. Yingling]

ARC provided by the publisher

In 1979, Theodora Underwood's parents are more interested in following their own scientific interests than in "Thunder", so she is tutored by a nanny in Boston during the school year and shuttled off to various relatives in the summer. She is used to being alone, and has a lot of initiative. When she tires of staying with a college aged cousins, she managed to travel by bus and taxi to Philadelphia to stay with her Aunt Hazel in her crumbling historical mansion in Fairmount Park. Thunder's parents consider Hazel "hippie-dippie", and certainly the organization of the house is more carefree than the methodical ways of the other Underwoods. She is very welcoming to her niece, however, and gives her her own room, specially decorated for her, as well as a notebook. Since most of her relatives were just annoyed to have to deal with her, Thunder is pleased. She is even able to have a storage closet for her own hideaway. When she finds a 200 year old letter there written by Margaret "Mercy" Percy, she carefully handles it, and when she tells Hazel about it, is allowed to keep it and investigate the mystery it outlines. The house had been owned by Benedict Arnold, and the letter mentions his wife, Peggy, and has cryptic clues to the whereabouts of Mercy's journal. Hazel gets help with the house from handyman Mr. Dixon, as well as Pop Jackson, who has two children, Charlie and Heather. Thunder isn't used to having friends, but gets drawn in to the Jackson children's plans, especially when they help her with the mystery. There is also a prowler in the area, purportedly looking for an important ring. After finding Mercy's journal, the children embark on a sometimes treacherous investigation to try to uncover the identities of women spies during the American Revolution. Thunder enjoys the hunt, and loves living with her Aunt and talking to people like Professor Maggie Kenner. As the summer draws to a close, however, will she have to return to being homeschooled?

Thunder is an engaging character who is having trouble understanding why any adults would find her company valuable. Aunt Hazel is delightful and welcoming, and her house, while it needs a lot of renovation, offers much scope for the imagination. There are hidden passages, very old documents, and other artefacts that are quite interesting. I appreciated that Thunder didn't keep too many secrets from Hazel, and Hazel gave her a lot of leeway. Watching Thunder interact with the children in the neighborhood was also interesting, and her desire to remain with a supportive and fun community is understandable, especially since her parents don't seem very pleasant.

The mystery is well developed, and there are a lot of clues and information about the Revolutionary War, which is especially nice during the Sestercentennial! This reminded me a lot of books I read in the 1970s, with plenty of details from the journal about goings on. Fortunately, the journal entries are in a different font that has some characteristics of cursive by is closer to print, so young readers can decode it Real historical characters are referenced, and readers who are interested might want to pick up Sheinkin's The Notorious Benedict Arnold for more information. There is some tension when neighbors are suspected, but I don't want to give away any twists and turns.

There are plenty of details about life in the 1970s, and popular movies and songs are mentioned frequently. Since I was about Thunder's age at this time, I can tell you they are pretty much spot on, although by this time I am not sure we would have been wearing an orange polyester pantsuits. That seems more 1975. Fashions changed incredibly quickly in this decade!

Readers who love history and are up to the challenge of 368 pages of history and clues, and who enjoyed Durst's Spy Ring or Landis' Capitol Chase, will look forward to this title by the author of Goodbye, Mr. Spalding.

https://msyinglingreads.blogspot.com/...

[Stephanie Tournas]

Thunder’s entomologist parents are away for the summer and once again she has been foisted on cousins who don’t want her. It doesn’t work out and the 11-year-old takes a bus from Boston to Philadelphia alone and lands on the doorstep of Aunt Hazel. For once, Thunder feels fully welcome. It’s 1979 and her aunt is a hippie-like woman with long hair and bangles, nothing like her buttoned up parents. Thunder, her name a nickname of her full name Theodora Underwood, sets about making herself a reading cave and exploring the 18th century house named Mt. Pleasant in which she finds herself. When she discovers a 200-year old secret letter with the word Help on it hidden under the floorboards of her closet cave, Thunder is determined to figure out who wrote it and why. Thus begins her epic historical journey into espionage during the Revolutionary War, taking her around the neighborhood of historic houses, into downtown Philadelphia on her bike, while trying to discover the prowler who may be on the hunt for the same thing she is.

The author sets the scene of 1970’s Philadelphia nicely, with language (“Neat!”, “Cool!, “Groovy!”), culture (Tiger Beat Magazine and lots of 1970’s music and TV shows mentioned) and loads of outdoor play that doesn’t involve screens. Two neighborhood kids, Heather and Charlie, become her unexpected friends and allies, the first friends the independent Thunder has really ever had. She impresses the historian Maggie Kenner at the University with her research into the letter writer Mercy, her name also a nickname for her full name Margaret Percy. As Thunder reveals more 200-year-old entries from Mercy’s journal, she delves deep into the drama of the American Revolution, finding that Benedict Arnold lived in the very house in which she is staying. Mercy seems to have been trying to get information to the patriots about Benedict Arnold’s treachery, and Thunder is determined to figure out if she was able to do so without being compromised. The story is a fun historical mystery, with suspense, friend drama and secrets from adults. I also really liked how Thunder modelled historical research. All characters cue as white except Maggie Kenner, the historian, who is Black.

[the_happyreader81]

Calling all educators, parents, and people who know middle graders! THIS BOOK IS A MUST READ!

As a middle grade teacher, I end up reading a lot of middle grade books in an attempt to keep my classroom library fresh, find new books to teach, and find books to recommend to my students who love to read, so please believe me when I tell you - this book is worth your time. Order it now, read it with your middle schooler, they'll love it, you'll love it. It's a win, win!

"Thunder and Mercy" follows our main girl Thunder (a super likeable, capable, yet flawed 11 year old) sent to live with her Aunt Hazel in Philadelphia for the summer while her parents are on a scientific expedition. While there, Thunder learns of a mystery involving her house, the American Revolution, and spies when she uncovers a note from 1780. As the mystery unspools, Thunder and her new friends Charlie and Heather learn about parts of history I was unaware of (note: I love it when books teach me about something I didn't know about and cannot wait to talk to students about the Culper Spy Ring!).

I think middle graders will love how much credit Barr gives them for understanding the history and various plotlines as they follow along with the story. She expertly weaves several plotlines (Thunder's frustration with her parents for leaving her, Thunder's trouble trusting, the mystery, Heather's struggle with a friend) and trusts the middle grade reader to follow. I think they can and will! This is NOT one of those books that hits them over the head with an obvious theme or moral again and again.

In fact, it's a book that asks the reader to really think about what it is trying to say. It asks the reader the question "How do you best solve big problems?" "When is it OK to forgive?" and for the educator "How does a reader really know when a character is changing?"

There is so much heart and intrigue written into the pages of this book I can see it being a new favorite of many who read it!

[Darla]

What do a preteen from 1979 and one from 1779 have in common?👭

More than you might have guessed. 😎

When I saw the publication date for this new middle grade historical novel was in July of this year, I was certain it would be a great time to learn more about Philadelphia during the Revolutionary War.

I was right. This story has so many things going for it. Here are a few:

🚕Theodora Underwood (Thunder) gets her first glimpse of Mount Pleasant from a yellow taxi. "We Are Family" by Sister Sledge is playing on the radio. Thunder is wearing an orange polyester pantsuit and her Aunt Hazel will soon declare, "What people. We're family."

😇Jennifer Robin Barr really caught my attention when her bio mentioned Charlies Angels. I don't know about you, but was one of my favorite shows back in the day. The kids in this book sometimes amuse themselves by acting out favorite plots. Thunder will soon have a poster in her favorite hideaway.

😎Late 70's cultural bits abound: running up the Rocky steps, Shrinkydinks, a Welcome Back Kotter lunchbox, Fleetwood Mac, a Billy Joel "Big Shot" tee, and so much more. I was fourteen again!

📚History comes alive as Thunder discovers the journal of a girl from 1779 named Margaret Percy (Mercy). She was the cousin of Benedict Arnold's wife Peggy Shippen Arnold. The entries detail actions of the Culpepper Spy Ring and Arnold's eventual betrayal.

📝Jennifer Robin Barr includes notes on the many historical places and people in the book and gives us a glimpse into what was real and what was manufactured for the flow of the story. I learned some new things from this book and that is one of my favorite things about historical fiction.

Happy Publication Day (July 14, 2026). Thank you to Calkins Creek and Edelweiss+ for a DRC in exchange for an honest review. All opinions are my own.

[Michele Niec]

Reading Thunder and Mercy by Jennifer Robin Barr felt less like picking up a book and more like stepping into someone else’s world, and realizing how much of it felt familiar. Her writing pulled me in right away. The details are so vivid that I could easily picture the setting, but more importantly, I could feel what the characters were going through. It never felt distant or “historical” in that disconnected way. Everything felt immediate, human, and real.

What I liked most was how naturally she blends history with storytelling. It didn’t feel like I was being taught a lesson, but I still walked away understanding more about the time period. (Did I find myself Googling events as I was reading because she piqued my interest? Yes, yes, I did.) The history isn’t just sitting in the background; it actively shapes the characters and their decisions, which makes everything feel more meaningful. At the same time, the story is easy to follow without losing depth, which I really appreciated.

I also found myself connecting with the characters in different ways. Thunder felt especially relatable - curious, questioning, and rebellious with good intentions. There’s something very human in that. In reading Mercy's account of her life, I found her to be strength personified in a time of American discourse.

By the end, Thunder and Mercy stayed with me more than I expected. It’s not just a story about the past; it’s a reminder of how people, no matter the time period, are shaped by the same kinds of fears, hopes, and choices. It left me thinking, reflecting, and honestly just appreciating the way a good story can connect us across time.

[Tammy Higgins]

Thunder and Mercy is a well-executed genre blend of mystery and historical fiction. As a librarian, I am excited to add this title to my collection; crossover genres do well in my library. And with the celebration of the 250 Anniversary of the United States, this will fit well into my planned displays for the upcoming school year.

The mystery balances a plotline between the 1970's and the 1770's. Rich setting details, both historical and from pop culture, bring each time period in Philadelphia to life. Readers will turn pages to figure out clues to the mystery Thunder uncovers in the floor boards of the Mount Pleasant mansion while learning about the history of the American Revolution, Benedict Arnold's betrayal, Washington's Culper Spy Ring and the mythic Agent 355. However, the book shines most in Barr's ability to create life-like characters that readers will connect with despite living in a different time. As she did in her debut historical fiction, Goodbye, Mr. Spalding, Barr crafts an endearing friendship story, fully-developed side characters, and a determined protagonist who learns to trust in a sense of belonging - something every middle grader seeks.

Excellent historical backmatter, and a story with intrigue and heart. Highly recommend.

[Eileen Ruvane]

What a fun ride! Featuring an exciting adventure, intriguing mystery, and a rich cast of characters led by the intrepid Theodora “Thunder” Underwood, who is sharp, fiercely independent, and impossible to resist. The dual mysteries kept me turning the pages, weaving the current day action of the late 1970s with the American revolutionary era. But it was the deep emotional heart beating beneath the story that stayed with me. Readers will be rooting for Thunder as she navigates the loneliness of absent parents and constant change to finally find connection, friendship, and a sense of belonging. History, mystery and heart… Highly recommended.