It is a hot day in Rocky Beach and the three detectives are getting a bit bored due to a lack of cases. While Bob is busy with the latest acquisition of the three detectives, a laptop with Wi-Fi, Peter would much rather go surfing. But that won't happen, as he has to give a presentation on gold doubloons and their transport to Spain for history class. Since they have nothing better to do, the three detectives head to the historical research institute, where they want to conduct some research on the topic. There, they not only meet Commissioner Reynolds, but also find an old family chronicle that leads them to the trail of a mysterious Inca treasure. Of course, the Three Question Marks set out to search right away. Among other things, they also embark on a diving expedition to a sunken gold galleon. When they then fall into the clutches of Panamanian criminals, they must realize that once again, things are not as they seem, as the gang possesses secret knowledge about old pirates, Spanish gold transports, and a mysterious oath.