Dr. Jose Calandre Garcia had been working on the theory of Mega- steam when he lost contact with his old friend Professor Poopsnagle. Determined to continue the work that Poopsnagle had originally begun, Garcia traveled to Australia in a hot air balloon in search of his old friend. Unbeknown to him, however, the wicked Count Sator had discovered his plan and sent his henchman Murk to shoot down the hot air balloon. Murk shot down the balloon, but the old doctor survived the crash and was rescued by some young children from a holiday camp in Secret Valley. When they heard the doctor's story of why he had come to Australia, they decided to help him in his quest to find the 6 minerals of Megasteam.

The only contact with Poopsnagle that remained was a telephone number. When Garcia called the number, he got a message machine. He left a message for his old friend to contact him at the Secret Valley holiday camp. Instead of Poopsnagle, Garcia was surprised when a young boy of 12 years old turned up instead. It was none other than Poopsnagle's grandson. Together they resolved to build a flying bus which was originally designed by Professor Poopsnagle to help them in their search for the minerals.

In a nearby deserted town, the children gathered all sorts of junk to help make the flying contraption. Luckily, they found the wreck of an old bus as the basis of the machine. With the help of the children, and following Poopsnagle's plan, the bus was eventually completed, and based on steam power it was able to rise into the sky and fly at will. Count Sator, however, sent another henchman called Willie Dingle to spy on the children, and within the ranks of the children he commissioned the young boy named Matt to keep him informed.

Poopsnagle's grandson Peter also had an ancient parchment that his grandfather had left him, and on the parchment was a cryptogram written in Latin. After the cryptogram had been decoded, it was only a matter of time before they found the first golden salamander. The golden salamanders had been buried long ago to indicate where each mineral could be found. After the first one was found, the cryptogram on it would lead the children and the doctor to the next one. What they had to do in each case was to decipher the cryptogram on each salamander. The first one led them to a cave not far from Secret Valley, where after being caught in a mine shaft and surviving a gas explosion, they were able to find their way down the shaft and into an unknown valley where they found the second salamander and the first mineral.

The cryptogram on the second salamander indicated that the children were to follow the arrow pointing towards the sea. Once they had taken off in the Poopmobile, and were traveling in the right direction, they found themselves caught in a gale. The storm buffeted the bus mercilessly, and after some time in which the doctor had great difficulty in keeping control, the bus eventually came out of the other side of the storm. Seeing an island, they decided to land and check to see if they needed to repair the bus.