When a strange circus rolls into Nod's Limbs, Edgar and Ellen find like-minded mischief makers among the escape artists and carnivorous plant-taming showmen. In an effort to prove themselves to their newfound peers, the twins embark on an adventure to recover three amber artifacts. Meanwhile, Mayor Knightleigh and his ilk are after Edgar and Ellen’s mansion and someone is after the mysterious balm spring hidden underneath. With the help of their beastly pet, Pet, the twins are able to unravel some of this mystery and recover the artifacts, but when they are betrayed by one of their Big Top confidant Ormond Heimertz, it leads to the loss of the mansion, the success of the Knightleighs, and mayhem at the main tent.
The book begins with the now widowed Pierre Knightleigh trying to set the abandoned Tower Mansion on fire, believing it to be cursed while his manservant Robbins tries to persuade him otherwise. A foreign man steps out of the mansion holding Pet in his arms. The man says that he is a collector of special properties and wants to but the Tower Mansion, Pierre acepts as he wants to get rid of Nod's "curse", he asks the stranger for his name, to which the man replies: Sigmund Heimertz.
In the present, Edgar and Ellen look through Nod's journals to find out what Pet is, what it wants and it's connection to the balm. Edgar, who was experimenting with the balm, set it on fire to see how it reacted to heat, causing it to explode and set the entire laboratory and Nod's journals on fire. The twins then see Heimertz with Pet and a pickax and flee the house thourgh the sewers and hide in Nod's Limbs Waxworks. Caravans and about thirty wagons with animals arrive to Nod's Limbs. A man shoots a crossbow bolt at the twins' backyard, near Heimertz's shed saying "We are arrived. Send forth the Ithune -B".
The next day, the twins find a bizarre circus surrounding the wax factory. Ellen then spots a banner that says: "WELCOME ONE AND ALL TO THE HEIMERTZ SEVEN-RING CIRCUS SHOW AND CARNIVAL". Soon after that, The Midway Irregulars tie them up and take them to the Big Top, where they hang them upside down from trapeze hoops, although Edgar easily frees himself and his sister.
The Midway Irregulars then leave to greet the guests and the twins witness Heimertz (Ronan)'s reunionwith his uncle, the Ringmater Benedict Heimertz, who leaves after he reminds him of his trial. An angry Heimertz then throws a metal pole in the air, which almost hits Edgar and Ellen.
While hiding from from Heimertz, Edgar walks into the escapist Ormond Heimertz's tent who too agrees that Heimertz is "unstable". Edgar decides he wants to become Ormond's apprentice and Ormond promises to make the twins join the circus if they can get three ambers for him that are hidden around the circus without anybody knowing about it. After finding the first amber, Edgar and Ellen sneak into Heimertz's tribunal, where they find out that their mansion is property of the Heimertz family. Heimertz escapes from the tribunal and Benedict decides to suspend it. The Midway Irregulars invite Edgar and Ellen to prank in the midway with them, Edgar goes with the acrobats Mab and Merrick and the flant tamer Phoebe Heimertz while Ellen goes with the Junior Ringmaster Imogen and the clown apprentice Gonzalo.
Ellen and Imogen arrange the Milk Jug Melee by glueing to bottom jugs to the table, making it impossible to knock them down. When Stephanie tries to play, she only succeeds on knocking one down. Furious, she tries to get Ellen arrested for robbery until Madame Dahlia, the plant tamer, intervenes. She gives Stephanie a prize, scolds Ellen and orders the twins and the Irregulars to go to her tent, where she tries to use her Nephentes Leviathos Gustav to scare Ellen into behaving by making her believe she was going to let Gustav eat her unless she behaved, although her plan fails since Ellen knows that Nephantes Leviathos don't eat humans. Madame Dahlia then tells the children the story of Mad Duke to teach them that greed is no game.