In class, Emily runs into Maddie Fernandez. Maddie has always held a grudge against Emily for no clear reason. Emily and James privately refer to Maddie as “royal fungus” due in part to her mushroom haircut. Emily notices that Maddie has changed her hair and compliments her new hairdo. Maddie in turn compliments Emily on finding the Poe book. Maddie adds that she would’ve found the book faster if she had entered the competition. Mr. Quisling works diligently on something during class and instructs the class to read silently and work independently. Emily and James whisper plans to sneak a peek at what Mr. Quisling is working on. Maddie overhears them and offers to go look herself, reporting back that he is working on a Book Scavenger printout. After school, Emily and James decide to follow Mr. Quisling.

Emily and James follow Mr. Quisling onto the bus and exit when he does, amid the skyscrapers of the Financial District. They nearly lose sight of him near an idyllic redwood grove among the office buildings. They find him in the grove and watch him discover a Book Scavenger book. He writes down clues from it and hides the book back where he found it. Emily and James recover the book—a copy of Tom Sawyer—and realize that it relates to the unbreakable code. Emily remembers that she once tried to find a clue in another copy of Tom Sawyer and wonders what the link is between the two copies.

The Phoenix watches Emily pocket the book. He resents how she and James got so much attention for solving Mr. Griswold’s game. He attributes their success to dumb luck. Emily’s discovery of the book ruins his plan for revenge.

Back at home, Emily and James discover that Tom Sawyer has already been marked as “found.” They wonder why Mr. Quisling would re-hide the book. They decide to hide the book again and see what Mr. Quisling does with the clue.

During class the next day, Mr. Quisling introduces a guest: aspiring substitute Harry Sloan. Emily recognizes Mr. Sloan from the book party. During quiet time, Mr. Sloan offers Mr. Quisling unsolicited advice about his teaching.

After school, they go to Hollister’s to hide the book. They meet Hollister’s new employee, a college student named Charlie with an aloof demeanor and big gauges in his ears. Emily mentions her interest in picking up a job, and Hollister agrees to hire her if her parents sign a permission letter. Emily hides the book in the store and asks Hollister about the unbreakable code. She learns that Hollister knew Mr. Quisling and that for years Mr. Quisling has been obsessed with the unbreakable code. Hollister remembers when he and Mr. Quisling worked on the excavation of the Niantic and revived interest in the code.

Hollister reveals that Mr. Griswold was heavily involved in the push to solve the unbreakable code. Emily is surprised that Mr. Griswold didn’t say more about it when she asked him. She worries about Mr. Griswold. They pull up an old interview with Griswold on the Book Scavenger website, in which he talks about the treasure hunt and why he finds it so personally inspiring. For him, it’s not about the outcome but about approaching the journey with curiosity and passion. Emily and James want to show the old interview to Mr. Griswold to help him remember.

Emily and James attend their first dance committee meeting. James has roped a few other friends into joining. Emily worries when Maddie shows up as the final member of the committee. Vivian asks for suggestions for the dance, which she has decided will be “President’s Day” themed, but she shuts down any suggestion she receives. Emily suggests combining everyone’s ideas into a Valentine’s-President’s-Day-Game-themed dance. Vivian hands out tasks to everyone on the committee, and Emily agrees to book Charlie as the DJ. Emily feels both a little proud and overwhelmed about speaking up in the group meeting.

A week goes by, and Mr. Quisling has yet to pick up on the Tom Sawyer clue. Emily looks up his Book Scavenger profile and discovers another user named Coolbrith, who has only ever hidden copies of Tom Sawyer. Mr. Quisling found every copy. Emily digs deeper and discovers a book quest, a game-within-a-game where players hide books back and forth using ciphers. Mr. Quisling refers to Coolbrith as “Miranda” in his public messages. Emily remembers Miranda from the picture of the Niantic dig.

They head to Hollister’s to take another look at the book. James and Emily ask Charlie to DJ the school dance, and he agrees after learning he will make $500. Emily and James look through other editions of Tom Sawyer and crack one of the other clues that led Mr. Quisling to the floral purse at the book party. They find a clue referring to a new map.

Emily and James decode the message from the Tom Sawyer they found in the redwood grove and discover that Miranda has abandoned Mr. Quisling in their quest. Emily feels for Mr. Quisling. Hollister trips over a table and wonders who moved it. Emily knows that Charlie moved the table and wonders why he doesn’t own up to it. Emily flips through one of the new books that spilled out, a book about San Francisco. The way the book is illustrated reminds James of a “grille cipher,” a kind of code where the decoder places a stencil over the text and certain words are emphasized. Emily thinks about what she has gleaned from following Mr. Quisling and decides to reach out to Coolbrith. Emily writes an email to Miranda Oleanda asking for the map on behalf of Mr. Quisling’s class.

During class, Emily feels sorry for Mr. Quisling, imagining that he is heartbroken. Emily and James try out different ciphers with no luck. Emily experiments with folding the page she copied from the code in the library and using the seemingly pointless doodles. When folded in half and held up to the light, the letter X lands inside a circle. Emily realizes that the code itself is a map. They recognize the three circles as the three islands in the bay. The treasure might be on Treasure Island.

Emily convinces her parents to take her and James to Treasure Island. Emily feels daunted by the island’s boarded-up buildings and isn’t sure where to look. She tenses up when her parents offer to pay for James’s bike rental and resolves to find the treasure for their sake. Emily learns that only part of the island is natural while the rest is man-made. Her heart sinks when she learns the island wasn’t built until the 1930s, well after Mark Twain and the Gold Rush.

Emily, James, and Matthew go to Bayside Press for their next advisory meeting. Mr. Griswold looks a little more like his old self. Emily shows him a printout of the interview she found. As soon as she brings up the unbreakable code, Mr. Griswold claims he has a vet appointment and leaves with the dogs. Emily worries that Mr. Griswold doesn’t want to return to his old self.

At the dance committee meeting, Devin and Kevin bring president costumes for the “presidential games” element of the dance. The kids try on silly costumes and choose their presidential teams. Maddie puts Emily on the spot to come up with a game for three teams to play. Emily comes up with the idea of a balloon stomp, where each team member has a balloon and opposing players try to pop each other’s balloons. James suggests adding a relay race element. Their pregnant teacher, Mrs. Ortega, starts groaning, noticing something with the baby. Maddie helps her while she calls her doctor.