Indeed, it appears as if Lord Hastings has finally found the legendary city of Guyanacapac and its immense riches somewhere in South America with the help of the local indigenous Moxtechica (Mox).

Desperate and pregnant from her lover, Vivian decides to join the expedition and visits Dr. Livesey in search for help.

Reluctantly, Dr. Livesey introduces her to the former pirate and cook Long John Silver whose help Vivian needs if she wants to take command of the expedition, thus preparing a mutiny.

However, as the map to Guyanapac was lost together with Captain Hastings in the mutiny, the crew has to rely on Mox's guidance across the meanders of the Amazonas river and its numerous effluents.

Not even yet arrived in Guyanacapac, the adventure takes a turn for the worse when the Neptune suddenly disappears shortly after finding what is left of the Nimrod, Lord Byron Hastings' ship, amid increasingly mysterious circumstances.