The series is a mix of ancient South American history, archaeology, and science fiction. The travellers encounter the Maya, Inca, and Olmecs during their journey. They discover many lost technological wonders of the Mu Empire, including a solar powered ship (the Solaris) and The Golden Condor, a huge solar-powered would-be ornithopter (mechanical bird), capable of traveling considerable distances under the sun's power alone. They are constantly pursued by antagonists Gomez and Gaspard, who are also in search of the Cities of Gold.

The Seven Cities of Gold were built by the Emperor of Mu over fear of a global war which would destroy all civilization. Such a war did break out, destroying the Empires of Mu and Atlantis when they used the "Weapons of the Sun" (implied to be thermonuclear weapons). The Seven Cities of Gold hold copies of books in their "Universal Libraries" as well as powerful artifacts, including the "Great Legacy", a portable fusion reactor. Other elements of this technology turn up in unexpected places, like the Solaris in Tao's home island, Esteban's and Zia's medallions as keys to the Cities, or Tao's jar as an important piece of the Great Legacy.