James Gray, whose previous films have all been situated in the environs of New York City, ventures much farther afield with his adaptation of David Grann’s book about Percival Fawcett, the British colonel who explored Amazonia between 1906 and 1925, first to map the border between Brazil and Bolivia and then in search of an ancient civilization he was certain had once flourished there. Though the picture represents a change of locale for the writer-director, however, it boasts the same technical proficiency as his previous work, as well as his essentially classical style. The result is an epic that is old-fashioned in a good sense. It is, however, also punishingly slow, and so restrained that it can at times feel a mite flaccid.

Fawcett (Charlie Hunnam) and his wife Nina (Sienna Miller) are introduced at a stag hunt where the gallant officer brings down the prey but is overlooked by the powers-that-be at the celebratory ball that follows because, as one swell remarks, he was unfortunate in his ancestors. (His father, we learn, had been a drunk and a gambler, and the family’s reputation suffered as a result.) Soon after, however, he is invited by the the Royal Geographical Society, which apparently had had dealings with his father, to undertake the mapping mission in Amazonia. Though he initially seems ready to decline, he accepts when it is pointed out that success would bring him a measure of renown—this is, after all, an era when explorers were celebrated in Britain—that could restore his family’s honor. (The mission is also important from a financial perspective, since hostility between Brazil and Bolivia could imperil lucrative European rubber interests in the region.)

Leaving Nina and his infant son Jack behind, therefore, Fawcett goes off with his army friend Henry Costin (Robert Pattinson, almost unrecognizable with a full beard) and a native guide called Willis (Johann Myers), and securing additional men from a German rubber plantation manager, he ventures down the river. At first the trip seems peaceful, but the group is soon attacked by spear-wielding natives, and further along he will encounter tribes that can be either hospitable or hostile. The group will eventually be forced to turn back, but not before Fawcett finds, deep in the jungle, shards of pottery and statuary that convince him a highly developed civilization once existed nearby.

When he presents his revolutionary ideas to a public meeting of the Society, however, most of the members, while applauding his achievement, ridicule the notion of South American culture that predated the emergence of European civilization. Only a few souls express support, the most notable being James Murray (Angus Macfadyen), a wealthy radical who vows to accompany him on another expedition. That one too ends in failure, not least because Murray proves unequal to its demands and must be sent back alone with no assurance that he will survive. He does, however, and later accuses Fawcett of abandoning him, a charge that takes a toll despite its falsity.

It is not that which delays Fawcett’s return to Amazonia: World War I intervenes, and he and Costin serve in the trenches together. During a gas attack his heroics at the battle of the Somme in 1916 leave him blinded, and he must be nursed back to health. Though now in his mid-fifties, he is prodded by Jack (now played by Tom Holland), who earlier had chided him for preferring his exploration to his family, to undertake another expedition, with his son accompanying him. This last venture ends in their disappearance, and though their fate remains unknown, Gray offers a speculative account of how the mission might have ended.

In telling this tale of the sort of exploration of unknown regions that held the western world spellbound at the turn of the century, Gray alters the record for dramatic effect. He reduces the seven expeditions Gray actually made to Amazonia to three, and certainly endows him with a more modern, enlightened view of the indigenous tribes than he actually held. Overall, however, he captures the quality of quiet obsession that appears to have driven Fawcett (along, of course, with personal ambition). It is, however, a restrained obsession that never takes on the mad quality that Werner Herzog, for example, portrayed in both “Aguirre: The Wrath of God” and “Fitzcarraldo.” (There is a hint of that, however—perhaps a nod to Herzog—in the scene in which Fawcett reaches the German rubber plantation on his first expedition: the manager there is putting on a performance of Mozart’s “Cosi fan tutte” in the middle of the wilderness. Fitzcarraldo would have approved.) That explains why Hunnam’s performance will strike some as entirely too buttoned-down. Despite occasional flashes of emotion, his Fawcett remains essentially calm and principled throughout, even when confronted by dangerous cannibals or jeering Royal Society members. While Hunnam’s portrayal might seem understated, though, it certainly reflects the portrait Gray wants to draw of Fawcett, and probably the stiff-upper-lip character of the man as he actually was.

The supporting cast is equally refined, with only Macfadyen’s volatile Murray—whose ultimate fate is also revealed in passing—bursting the bounds of the social propriety of the time. Miller cuts a radiant figure as the supportive but self-confident Nina, who wants to go to the Amazon too, while Holland brings his engaging boyishness to Jack and Myers a sense of deep dignity to Willis. Then there’s Pattinson, who utterly sheds his erstwhile heartthrob image to play Costin as an almost drably ordinary fellow who will follow Fawcett virtually without demur—until he gets married and has family commitments he’s unwilling to ignore. A large contingent of fine British character actors use their skill to sketch the lesser figures with admirable economy.

Working on what was probably a non-epic budget, production designer Jean-Vincent Pezos and costumer Sonia Grande provide elegant period accoutrements and cinematographer Darius Khondji captures the well-chosen locations (the jungle scenes shot in Colombia) in lush widescreen images. A traditional score by Christopher Spelman complements the visuals nicely.

The patient, admiring approach Gray adopts in telling Fawcett’s story is itself admirable, but the patience it demands of viewers may be too much for some. “The Lost City of Z” is beautifully crafted and, like its subject, high-minded; it is not, however, exciting, as one might expect a tale of exploration of a dangerous environment to be. It’s an impressive piece of work, but at times one wishes Gray had taken a few risks—after all, Fawcett certainly did.