Dan, Amy, and their au pair Nellie, found music by Wolfgang Amadeus Mozart in the end of The Maze of Bones, leading them to Vienna, Austria, to learn about him and find a related clue. In Vienna, Amy and Dan discover that Mozart had an older sister: Maria Anna "Nannerl" Mozart. They go to a library to view her diary, only to realize Jonah Wizard, a fellow competitor in the search for The 39 Clues, stole it. They steal it from him, but then Nellie translates it and notices that three pages are missing. After finding the music from Mozart on the Internet, Amy and Dan notice that three lines are missing from it. They play the missing lines down in the lobby of the hotel they are staying in and realize that they are actually a whole different song that wasn't even written by Mozart. That song's name is "The Place Where I Was Born", so they go to the place where Mozart was born: Salzburg, Austria. There, they see Alistair Oh, and follow him into the Salzburg Catacombs. They see the man in black and, shortly afterward, are trapped inside the Catacombs by an explosion that causes a cave-in. However, they find another way out through St. Peter's Archabbey and are chased by monks after finding a sheet of old parchment that supposedly had all 39 clues on it. They are devastated to find that it is just a recipe for Benedictine. Later, Nellie discovers that there is a homing device on the collar of their cat, Saladin. She, Amy, and Dan then find Alistair sleeping on a park bench and decide to plant it on him. Amy finds a secret compartment inside his cane and plants the homing device inside, in exchange taking what he found in the Catacombs, an eighteenth-century concert poster starring Mozart in Venice, Italy. In Venice, Amy and Dan follow Jonah Wizard and find a secret passage from a music store called "Disco Volante" to a Janus stronghold. There, they find the missing diary pages and steal them from Jonah while he is examining them. They are chased by Janus agents but hide the pages on a boat called the Royal Saladin to come back to collect them once they lose the Janus. The pages say that Nannerl thought her brother was going crazy because he was buying large quantities of an expensive Japanese steel and getting himself into major debt. A name, Fidelio Racco, which was also found on the paper taken from Alistair Oh, appears in the diary, along with two notes from Grace: "The word that cost her life, minus the music" and "D>HIC". They figure out that "the word that cost her her life" was referring to Marie Antoinette's famous quote, "Let them eat cake.", when she was told the peasants had no bread. Amy recalls from a conversation with Grace that Marie Antoinette used the most common French word for cake, gateau instead of brioche which is what she is usually quoted with. However, they do not know what Grace means by "minus the music" or "D>HIC", so they go to Fidelio Racco's mansion and hide until after it closes. They then sneak over to Fidelio Racco's harpsichord but are ambushed by the Kabras, who have been following them since Paris. Ian plays Mozart's music on the harpsichord but is unaware of the booby trapped D key, which Amy realizes the meaning of D>HIC, meaning D over High C. She tries to knock Ian off the bench, but she is too late. His finger brushes the booby trapped key, and an explosion sends them both flying into the air. Amy manages to tuck and roll when she hits the ground, but Ian whacks his head on the marble floor and is knocked unconscious. Natalie is also knocked out after Dan stabs her with a dart from her tranquilizer dart gun. Most of the harpsichord is vaporized in the explosion, but the keyboard is still intact, so Amy plays "The Place Where I Was Born". A section of the floor drops down, revealing two Japanese swords and the second clue, . Amy figures out that gateau minus the music means that she needs to take out all of the letters that are musical notes, which leaves her with T-U, the chemical symbol for tungsten. Back at their hotel in Venice, Amy and Dan tell Nellie about the second clue, and she calls Japan Airlines to book three tickets to Tokyo. The book ends with Alistair Oh finding out who is owner of the tracking device that was placed on Saladin and himself. That person is Grace's lawyer and friend, William McIntyre.
Critical reception
reviewed the book, claiming that a reader may find inspiration in their own life and that "Gordon Korman...has done an amazing job."
