Three Hyde Park residents receive identical anonymous letters asking for help solving a centuries-old art mystery.

Calder follows Petra to Powell's Bookstore and the two bump into each other, beginning their unlikely friendship.

Petra finds Charles Fort's book Lo! in the giveaway bin outside Powell's Bookstore and brings it home.

Ms. Hussey takes the class on a field trip to the Art Institute to find works of art that contain letters.

On Halloween, Calder recognizes Petra's costume as the woman in Vermeer's painting A Lady Writing.

Calder and Petra call the National Gallery and learn that A Lady Writing is currently in transit to Chicago.

A Lady Writing is stolen before it arrives at the Chicago exhibit.

The thief sends a letter to the Chicago Tribune claiming the theft was meant to expose authenticity questions about Vermeer's paintings.

Ms. Hussey is arrested on suspicion of involvement in the art theft, then released after admitting she received one of the anonymous letters.

Petra and Calder find a wrapped parcel in the school basement but unwrap it to reveal only a crude amateur painting.

Calder realizes he and Petra share a December 12 birthday, both turning 12, and concludes the entire mystery hinges on the number 12.

Calder climbs to the 12th stair in Delia Dell Hall and pries open the 12th wall panel, revealing A Lady Writing hidden inside.

A man in a dark jacket chases Calder and Petra across the school playground after they escape the building with the painting.

The thief steals A Lady Writing from the back of a patrol car while Petra and a policeman search the playground for Calder.

Petra finds Calder unconscious in a neighbor's treehouse clutching the painting; the thief is later found dead of a heart attack on a train.

Three anonymous letters arrive in Hyde Park, setting a mysterious art investigation in motion before anyone knows a crime has been committed. Calder and Petra meet by accident at Powell's Bookstore and begin bonding over a shared fascination with coincidences and art, especially after Petra discovers Fort's book Lo! and Calder connects his grandmother's box to a painting by Vermeer. On Halloween, Calder recognizes Petra's costume as the subject of A Lady Writing, and the two soon learn the painting is traveling to Chicago — only to discover it has been stolen before it arrives. The thief's letter to the Chicago Tribune, Ms. Hussey's arrest and release, and a failed search of the school basement all deepen the mystery before Calder's epiphany about the number 12 leads them to the painting hidden inside Delia Dell Hall. A terrifying playground chase ends with the painting stolen again from a patrol car, but Petra ultimately finds Calder unconscious in a neighbor's treehouse with A Lady Writing clutched in his arms — and the thief is found dead on a train.

The core mystery: a stolen Vermeer and two unlikely detectives.

Ms. Hussey assigns students to find someone who has received a truly extraordinary letter, framing the novel's central motif from the start.

Petra overhears her parents arguing about a mysterious letter and searches the trash hoping to find it, but someone has already taken it out.

Petra has a vivid vision of a woman in old-fashioned dress holding a quill pen while drifting off to sleep.

Calder recalls his grandmother's decorative box showing a robed man holding a compass, with the letters 'Meer' visible on a background cupboard.

Calder receives a coded pentomino letter from Tommy reporting that a neighborhood boy named Frog has mysteriously vanished.

Calder notices Ms. Hussey, Mr. Watch, and Mrs. Sharpe all speaking together outside his house on the same afternoon.

Calder visits Mrs. Sharpe's home to deliver books and recognizes the print on her wall as matching his grandmother's box; she identifies it as a Vermeer.

Calder notices repeated objects across Vermeer's paintings and suspects the artist may have hidden coded information in his work.

Calder and Petra seal a secret pact over blue M&Ms, committing to work together on their growing Vermeer investigation.

Calder and Petra watch Ms. Hussey have a tense conversation at Powell's Bookstore and leave carrying a thick package under her arm.

Calder sneaks through Mrs. Sharpe's upstairs rooms during a book delivery, searching unsuccessfully for the stolen painting.

Calder decodes a letter from Tommy revealing that Old Fred, Tommy's stepfather, has abandoned the family in New York.

Petra and Calder spend an entire weekend organizing a bake sale and fundraiser to help Tommy's family afford the move back to Hyde Park.

Petra finds a fragment of one of the thief's anonymous letters sticking out of the hedge beside Mrs. Sharpe's house.

Authorities reveal the dead thief is Xavier Glitts, who married Tommy's mother under a false name to embed himself in the Hyde Park community.

Ms. Hussey's early classroom assignments about extraordinary letters quietly plant the seeds of the investigation, while Petra's failed search through her family's trash hints that the mystery is already closer to home than she realizes. Calder's grandmother's box reveals a painting connected to Vermeer, and a coded letter from Tommy about a missing neighborhood boy named Frog introduces a seemingly unrelated subplot that will eventually tie directly back to the theft. After Calder spots Ms. Hussey, Mr. Watch, and Mrs. Sharpe gathered suspiciously outside his house, he visits Mrs. Sharpe and first learns the name Vermeer, and the children seal their partnership over blue M&Ms before watching their teacher leave Powell's with a mysterious package. Coded messages from Tommy reveal that his stepfather Old Fred has abandoned the family, prompting a bake sale fundraiser, while a letter fragment found in Mrs. Sharpe's hedges suggests a fourth letter may exist. All loose threads converge in the final revelation that Old Fred is Xavier Glitts, an international criminal who married Tommy's mother under a false name specifically to infiltrate the Hyde Park community and orchestrate the theft.

The clues, side threads, and suspects that surround the central mystery.

Calder's best friend Tommy moves to New York with his mother and new stepfather, leaving Calder without his closest companion.

Petra reads Lo! and becomes captivated by Fort's argument that all events are interconnected and nothing is mere coincidence.

Calder sits with Petra at lunch for the first time and the two begin openly connecting the strange coincidences accumulating around them.

Calder learns that Lo! originally belonged to Mrs. Sharpe and volunteers to question her about it when he next delivers her book order.

Mrs. Sharpe learns Petra has her copy of Lo! and invites both children to come to her house for tea.

Calder's father casually reveals that Mrs. Sharpe's husband was a Vermeer scholar who was murdered in Europe decades earlier.

Ms. Hussey's class debates the thief's claims and most students conclude that Vermeer did not paint all works attributed to him.

Petra visits Mrs. Sharpe in the hospital and confesses that the Lady in the painting seems to be communicating directly with her.

Mrs. Sharpe demonstrates surprising skill with Calder's pentominoes during the hospital visit, spelling out a series of cryptic words.

Tommy sends word that Frog has resurfaced and that the postcard Frog mailed from the National Gallery bears an image of the stolen Vermeer.

Petra notices shapes in Delia Dell Hall's wrought-iron railing that mirror Mrs. Sharpe's pentomino words and announces she thinks they have found the painting.

The thief announces in a newspaper ad that he will destroy A Lady Writing if art authorities do not correct disputed attributions by his deadline.

Petra spots her father inexplicably carrying a rectangular parcel inside Delia Dell Hall, then watches him leave the building without it.

Calder climbs the playground slide and threatens to destroy the painting to buy Petra time to escape with the canvas.

Mrs. Sharpe tells the children that something far more powerful than any of them deliberately chose them to find the painting because they alone could see patterns others ignored.

Tommy's move to New York is the quiet catalyst that changes everything, leaving Calder without his best friend and opening the door for his partnership with Petra. Petra's reading of Lo! gives both children the intellectual framework to see connections where others see coincidence, and Calder's discovery that the book once belonged to Mrs. Sharpe draws them into her orbit and eventually toward Vermeer. Revelations accumulate — Mrs. Sharpe's husband was a murdered Vermeer scholar, the class largely sides with the thief's authentication claims, and during a hospital visit Mrs. Sharpe proves surprisingly skilled with pentominoes and warns the children that looking and seeing are two very different things. A postcard from the missing Frog bearing an image of the stolen Vermeer, Petra's eerie recognition of pentomino shapes in a staircase railing, and the thief's deadline threat all push the investigation to its breaking point. Petra's bewildering sighting of her father with a rectangular parcel inside Delia Dell Hall, Calder's act of sacrifice on the playground slide, and the final quiet revelation from Mrs. Sharpe — that something far larger than any of them chose Petra and Calder specifically because they could see what others could not — bring the story to its emotional and thematic close.