In The ClueFinders 3rd Grade Adventures: The Mystery of Mathra a great city was built 1000 years ago in the Numerian rainforest until a monster named Mathra invaded. After Mathra was captured, the Numerians abandoned their city and sealed the entrance and hid the two halves of the key in the far corners of the rainforest - one in the Monkey Kingdom and the latter in the Goo Lagoon. Recently animals have started to disappear in the rainforest once again, along with Joni's uncle, Dr. Pythagoras. Mr. Limburger flies the ClueFinders in his airplane and briefs them on the events going on. The ClueFinders set off to find the lost doctor, animals, and the keys to the Lost Numerian City. Evidence that they find, however, suggests that there is more to those disappearances than the 1000-year-old monster as well as a sinister plot behind it.