In the early 20th century, intrepid British explorer Lily Houghton enlists the services of wisecracking steamboat captain Frank Wolff to guide her through the Amazon jungle. Her mission: to uncover an ancient tree with unparalleled healing abilities—possessing the power to revolutionize medicine. Accompanied by her reluctant and well-mannered brother MacGregor, Lily's relentless pursuit of the mythical tree thrusts the unlikely trio into a labyrinth of supernatural wonder and imminent danger.

As they embark on their river voyage, the band of adventurers encounters countless hazards, from wild animals and treacherous rapids to the devious pursuit of rival German explorers, who vie for the same prize. While battling these external threats, Lily and Frank also match wits and banter, their chemistry growing amidst the perils. Their journey becomes a race against time, as both the natural elements and malevolent forces conspire to thwart their quest before the tree's miraculous secrets can be unlocked.