Tom receives a message from his missionary friend whom he saved from captivity in Africa during the adventures of the preceding volume.

The message describes a wonderful underground city, filled with treasures of gold, somewhere deep in the heart of Mexico.

Around this time, Andy Foger and his father had lost their fortunes and are off after Tom's trail in order to steal the treasures from him.

In order to make the trip possible, Tom must remodel his previous airship- a hot air balloon with an enclosed cabin.

In Mexico, they hire a team of Mexicans who catch onto the city of gold plot and chase after it in competition with Tom as well as the Fogers.

Before the others can explore the city, an underground river floods it and they make off with a huge wealth of salvaged gold.

The central gold statue has a warning carved on the pedestal it sits on in "the ancient Greek or Persian language".