They long to cross uncharted seas and discover unknown countries, to find secret gold on a mountain trail high in the Andes.

They are joined on their quest by Zia, an Incan girl (who was kidnapped by Gomez, Gaspard, Perez and Mendoza), and Tao, the last descendant of the sunken empire of Mu (Hiva in the English version).

[2] The Seven Cities of Gold hold copies of books in their "Universal Libraries" as well as powerful artefacts, including the "Great Legacy", a portable fusion reactor.

He possesses an encyclopedia which contains information about their lost technology as well as a mysterious jar which according to the legend, only the High Priest of the City of Gold can open, and it proves to be the Great Legacy's cooling or control rod system.

A world-threatening meltdown is avoided by the personal sacrifice of Esteban's father who, acting as the High Priest of the Cities, is presumed to be dead after he replaces the jar.

Mendoza sneaks Esteban aboard the sailing ship Esperanza, and once in the hold, he meets a kidnapped Inca girl named Zia who has a sun medallion nearly identical to his own.

Battered by the typhoon, the Esperanza breaks up and Captain Perez and the rest of the crew abandon ship leaving Mendoza, his two associates Pedro and Sancho, and the children to fend for themselves.

There, Zia translates a message in a knotted string called a quipu which leads to events that releases the Solaris – a golden sailing vessel.

The children set out to find Puna, Zia's home village, but to get there, they must go through a pass blocked by Pizarro's fortress which is built atop an underground temple.

Pasha reveals a secret escape route through a puzzle room, which once solved, melts a pillar of ice that collapses the temple and likewise brings down Pizarro's fortress.

Mendoza secretly tips off Esteban to the plan and Tao builds a giant flare which lights up the night sky and helps the Incas drive off the invaders.

Tao figures out that the marker points the way to the first city of gold, but before they can travel there, Esteban needs to have the sun symbol part of his medallion which Mendoza possesses.

The children meet back with Mendoza and set out to find the first City of Gold, but they come under attack by a hostile tribe of giant men called the Urubus.

After escaping they find an injured girl named Lana, who takes them to her village which floats on an island of reeds upon the waters of Lake Titicaca.

Koruka makes a deal to never attack the village again and provides a clue to reach the City of Gold – a place where Pachamama is said to stand guard.

Eventually, the light of the setting sun leads them to a statue of Pachamama, and placing Esteban and Zia's medallions onto the goddess' breasts triggers a wall to open which reveals the first City of Gold.

The gold is only an illusion created by the glow of the setting sun on ordinary stone, but the children discover another treasure – a massive golden condor hidden inside a great temple.

The next morning, the children examine the Nazca lines while Pedro and Sancho fiddle with the condor and discover a control stick in the shape of a cobra that can manually steer the bird.

The Amazon Queen praises Esteban for bringing out the sun, but Amoro still demands a sacrifice and chooses a young servant girl, named Morka.

The children learn that Amoro uses a sacred "rain mask" to foretell storms, but Tao discovers that the device is really a barometer made with knowledge from the Hiva people.

The children return the next morning to the room with the hieroglyphs and Tao finds writing hidden inside the eyes of the jade mask which leads them to a secret burial chamber.

Returning to the city, the children find that Mendoza, Pedro and Sancho have been taken prisoner by an unscrupulous Spanish explorer named Dr. Fernando la Guerra, and his assistant Marinche.

The writing gives clues to reflect sunlight upon the statue which opens a secret passage to a room containing an idol holding a second manuscript.

Esteban however, notices that Tao's wet manuscripts now reveal hidden symbols that spell out another message; a city of gold lies beyond a mountain called The Burning Shield.

Menator orders his craft to land and his men rush out to search the Condor for survivors, giving Esteban and Tao a chance to slip inside and overpower the Olmec pilots.

They are found by a mysterious high priest with a golden mask who claims that prophecy foretold of their coming, for they bear the medallions of the sun – the keys to the Cities of Gold.

When a gap is opened, Mendoza rushes inside and captures Kalmec setting Tao free, but once the gates are destroyed, Menator's machine flies in and reforms itself.

Earthquakes shake the City of Gold, and the high priest warns that the Olmec reactor must be shut down or it will burn a hole straight into the Earth.

The main cast of the series dubbed into American English, includes: The story was written by Jean Chalopin and Bernard Deyriès, and is very loosely based on the novel The King's Fifth by Scott O'Dell.

In contrast, Bernard Deyriès recalled his reaction to the proposed music he heard from Ulysses 31 in that he felt that the score was rather understated as he was expecting a more adventurous feel, something akin to films like Indiana Jones.