At the Fort of the Black Eagle , the main concern is the defense of the High Peak. Mendoza thinks that Gomez and his men will launch a new attack and asks Esteban to make it clear to Upanki that he must mobilize his troops . That's when Gomez appears. Mendoza warns him that he will not hesitate to bombard him with the cannon he has stolen. After four missed shots , Gomez, Gaspard and the Spanish troupe rush towards the fortress knowing that Mendoza only had four shots. The Incas replicate with their bows and their spears.
Tao has an idea. He notices that there is still a barrel of powder. With the help of small earthen jars , the children make bombs that they can send on the Spanish troop with the help of the cannon. The fight between the Spaniards and the Incas is raging but they take the advantage thanks to the invention of Tao. The Inca warriors congratulate the children to the chagrin of Sancho and Pedro who admit to feel useless.
For his part, Gaspard finds a small piece of clay on the floor and deduces the idea of ​​Tao. Gomez thinks that their supply of powder is exhausted and decides to relaunch the attack against the fortress. Upanki sends Esteban to the High Peak to report to Kraka about the situation. Mendoza supports this idea and urges children to leave . Faced with this desperate situation, the chief of the Fort garrison decides to withdraw to the High Peak.
At the same time, the door gives way and the Spaniards enter the Fort. Gomez orders Gaspard and two of his men to go after the children. These discover the entrance of a cave in the mountain and penetrate quickly . Alerted by Kokapetl , Gaspard and his men follow them closely. Along the underground tunnel, the children are guided by a phosphorescent light emanating from the walls of the cave. Gaspard's strong voice awakens a colony of vampires that delays the pursuit of children and allows them to take a little more advance. Esteban, Tao and Zia arrive at the end of the cave and find an underground river . Tao notes that this small body of water is crossed by a slight current and deduces that this river must flow "somewhere". They decide by mutual agreement to follow the direction of a glow they see in the water and plunge courageously . Gaspard also joins the dead-end and presumes to have gone astray. He decides to turn back and get out of this grim cave. The children continue their swim towards the light seen in the distance by catching their breath in a pocket of air, testifying to an old inca secret passage. Listening only to their courage, they swim until they are carried away by a swirl and finally thrown on rocks next to a waterfall . All three regain consciousness and realize to have been saved by the Sun ...