Adapted from David Grann’s 2009 work of non-fiction, The Lost City of Z follows Percy Fawcett (Charlie Hunnam), a British military man-turned-explorer who first ventured into the Amazon rainforest at the turn of the century. At first, Fawcett was dispatched as a surveyor, but eventually he became convinced there was evidence of a lost civilization hidden in the jungle, one as technologically advanced as any in the ancient world. Accompanied by a salty aide-de-camp, Henry Costin (Robert Pattinson), and, later, by his own son Jack (Tom Holland), Percy returned again and again to the Amazon in search of evidence he could bring home, flying in the face of then-held beliefs about the intellectual limits of “primitive” societies.

I haven’t gravitated toward Hunnam as an actor in the past, since he’s so often slotted into handsome leading-man roles entirely lacking in dimensionality (as in Pacific Rim). But as Percy, he’s sensational. His character is driven and haunted, but not insane or unfulfilled, given his happy marriage to Nina (Sienna Miller) and his obvious love for his three children. Hunnam portrays Percy’s fixation on the Amazon as something that’s not easily dismissed: A mix of ego, a desire for fame, and genuine intellectual fascination keeps pulling him back into a life of danger and long separation from his family.

His repeated trips find him boating down the Amazon river on simple rafts with Costin and other local guides, navigating the complicated political landscapes of Bolivia and Brazil as the region is ravaged by rubber tycoons, warring colonial powers, and the creeping threat of industrialization. Gray makes it clear that Percy is a white invader in a land he doesn’t understand, while recognizing that his contentions—that Amazonian societies had farmed the earth, built complex structures, and created pottery and art—were seen as absurd and borderline offensive in Great Britain.

The notion that such advanced civilizations could have existed in South America was widely dismissed in the West, but Percy had an empathy for the region that was unusual. While others are seeking to strip the Amazon’s resources away, he wants only to witness its ancient artifacts. Industrial-scale expeditions bring guns into the jungle to do battle with local tribes, but Percy simply tries to reason with them, looking to find common ground so that they can help him search more deeply into the jungle.

Gray captures all of these dynamics with appropriate subtlety. He doesn’t dismiss the danger that Percy and his crew face around every river bend, or inadvertent harm they could do to the rainforest by attracting more Westerners with their discoveries. But he also emphasizes that Percy’s fascination with the region borders on the religious, as if discovery of these ancient wonders will finally answer some formless question gnawing at his soul. How else to comprehend the mind of the explorer who returns to the Amazon over and over again with little more than a pack full of food and a compass?