He cried in a whisper at some image, at some vision - he cried out twice, a cry that was no more than a breath - "The horror! The horror!"5976-1: River Expedition is clearly The LEGO Group's take on the Joseph Conrad novel Heart of Darkness. Johnny Thunder takes on the role of Charles Marlow, captaining the ship down the Congo river, delivering ivory to his Belgian employers.
Kurtz is the man with the mask atop the temple wielding snakes of course. Really interesting that one could easily view it as a Heart of Darkness inspired set.
I guess it does bring up the issue of why Johnny Thunder and Dr. Charles Lightning are exploring this dense, dark river. The obvious answer is some sort of National Geographic expedition, with cameras to record their findings. But I wonder if there is more to it. Are they seeking to appropriate the riches of the jungle for themselves, stealing from the natives?
Probably not. Throughout many of the adventurers series there are three groups. Thunder and his companions, the natives, and a third group of villains. The villains generally change depending on the subtheme (Jungle subtheme introduced Senor Palomar, for example) and are usually trying to steal the treasure for themselves. So it would appear that Johnny Thunder has no ill intentions, but rather is a genuine explorer, seeking knowledge.
However, the newest Adventurers subtheme, Pharaoh's Quest, does not feature any video equipment nor any villains. In fact, the main protagonists are the ones raiding the tomb (below). An interesting development that should be investigated further.

