Synopsis
A thousand years ago the ancient Numerians, great inventors, built a great city in the Numerian rainforest. According to legend, a monster named Mathra appeared out of nowhere one night. Mathra attacked the wild animals of the jungle, but the Numerians managed to capture Mathra. Fearing the monster might escape again, they abandoned their city, never to return. They locked the door to the city, split the key in two, and buried the halves in puzzles "in two corners of the rainforest." The Numerians still live in the villages around the Numerian rainforest, but the location of the Lost City remains a mystery.
When animals began disappearing in the Numerian rainforest again, some people suspect Mathra responsible for the animals' disappearance. Dr. Pythagoras, however, refuses to believe that, instead believing there should be a simple logical explanation. However, before he could explain, the doctor is suddenly kidnapped by what seems to be Mathra. Dr. Pythagoras' last words are, "Call the ClueFinders!" Interestingly enough, Joni is Dr. Pythagoras' niece.
The ClueFinders fly to the rainforest in Mr. Limburger's airplane, where he explains the legend of Mathra to them. The ClueFinders conclude that they need to get to the Lost City. Joni and Santiago set off to find the keys and Owen and Leslie stay behind. They also join LapTrap, Dr. Pythragoras' Turbo TURTLE, who has a map of the rainforest that comes in useful.
The two areas of the rainforest that contain the keys are the Monkey Kingdom and the Goo Lagoon. In both places, they meet highly intelligent animals and plants which can communicate to each other and the ClueFinders. While searching each area, Joni and Santiago find clues that suggest that Mathra couldn't be behind the animals' disappearance: a badge labeled "Fletcher's Furriers" and a can of aviation engine oil.
After Joni and Santiago collect both key halves, Mr. Limburger offers to fly them to the Lost City. However, when they jump out of the plane with their parachutes, they land in the jungle canopy, far from the Lost City. The ClueFinders eventually make their way on their own to the Lost City's gates. When they take out both keys, they fuse together to form one key, which they then use to unlock the gate.
Inside, they find what appears to be Mathra sleeping, but then it occurs to them who is really behind the animals' disappearance — Mr. Limburger. Limburger built a disguise for his airplane, so it would appear to be Mathra, having used the legend of Mathra to provide a cover of his smuggling of animal hides for his fur company. It turns out that "Fletcher" is Limburger's first name. He reveals that kidnapped Dr. Pythagoras because he felt that Dr. Pythagoras was too close to exposing the smuggling operations, and is now being held with the animals that he had captured. After admitting that he was behind the mystery, Limburger locks Joni, Santiago, and Laptrap in the city and flies away in his Mathra plane to collect more animals.
Eventually, thanks to the ancient talking guardians of the Lost City, the ClueFinders discover that Dr. Pythagoras is trapped on the other side of the Bottomless Pits of Doom with all the animals that Limburger has been collecting. They rebuild the Mathra trap by collecting snakes made from anicent Numerian magnets (known as 'snagnets'), which the Numerians used to trap Mathra, and rescue Dr. Pythagoras and the animals in the process. After they do so, the Mathra trap becomes invisible, and the Cluefinders lure the returning Limburger to fly his plane into it. Upon falling into the trap, Limburger falls down the pit with his plane.
When the ClueFinders leave the rainforest by boat, as a reward for them for saving the rainforest, Dr. Pythagoras gives them LapTrap, who faints when he hears he'll be going on more dangerous adventures with them. Vasco da Bongo and the Four Flowers wave goodbye to Dr. Pythagoras and the ClueFinders. Meanwhile, Limburger reappears in the rainforest, having survived the plunge on the bottomless pits and now left with his clothes torn. He swears his vengeance against the Cluefinders, and as he walks away with his dark mood unappeased, the real Mathra appears behind him, roars, and spouts fire, hinting that Mathra could be a dragon.
Read more about this topic: The ClueFinders 3rd Grade Adventures: The Mystery Of Mathra