In 1905, a young British major named Percy Fawcett takes part in a stag hunt on an Irish baronial estate, organized for the visiting Archduke Franz Ferdinand. Despite his adept skills as a horseman and marksman—efficiently bringing down the stag—he receives little recognition at the after-hunt party.
Fast forward a year, Fawcett finds himself in London, meeting with officials from the Royal Geographical Society (RGS). They're informed about potential conflict between Bolivia and Brazil over their shared border, which poses a significant threat to the region's highly profitable rubber trade. The British government has been asked to conduct a survey of the area, giving Fawcett the perfect opportunity to lead his own expedition.
Teaming up with Corporal Henry Costin—who has experience navigating the Amazon rainforest—Fawcett embarks for Brazil. At a expansive rubber plantation owned by Baron de Gondoris, they encounter Lance Corporal Arthur Manley, who warns against their exploration ambitions. Ignoring the pessimism, Fawcett, Costin, and their native scout, Tadjui, press forward to complete their survey mission.
While on their journey, Tadjui shares intriguing tales of a legendary golden city hidden deep in the jungle, a notion Fawcett initially dismisses as mere folklore. However, his skepticism fades when they discover refined yet broken pottery and intricate stone sculptures, suggesting that there may indeed be truth behind the legends.
Upon his return to England, Fawcett is celebrated as a hero, and his wife, Nina, has just welcomed their second son into the world. In an intriguing twist, Nina uncovers an ancient text in the Trinity College Library, detailing a similar city that Fawcett names "the Lost City of Z." Motivated by this discovery, he seeks funding to undertake an unprecedented expedition, eventually finding backing from renowned biologist James Murray, despite facing initial ridicule from the RGS members.
Nevertheless, Murray's unfamiliarity with the harsh realities of jungle exploration hampers their progress. Amidst escalating tensions, their party comes under attack along the river, forcing Fawcett to broker peace with local tribes. Unfortunately, Murray suffers a debilitating leg infection that spirals into madness, leading Fawcett to send him away in hopes of obtaining help. The team eventually has to abandon their mission after discovering that their supplies have been spoiled.
Controversy ensues when Murray survives and accuses Fawcett before the RGS trustees for allegedly leaving him behind in the jungle, demanding an apology that Fawcett refuses, ultimately leading to his resignation from the society. When World War I erupts, Fawcett finds himself fighting in France, witnessing the loss of Manley at the Battle of the Somme and experiencing a life-altering chlorine gas attack that leaves him temporarily blind. It is during this time that he reconciles with his estranged son, Jack, who had long held resentment toward his father for abandoning the family.
By 1923, Fawcett has faded into obscurity back in Britain. Meanwhile, American interest in the Amazon has surged, largely due to Fawcett's captivating tales of the lost city. With backing from John D. Rockefeller Jr. and U.S. newspapers, a new expedition is organized, with the RGS reluctantly agreeing to co-fund it as a point of national pride. Before they depart, Fawcett confidently shows Sir John Scott Keltie a compass, vowing to return it once he discovers the fabled city.
This time, Fawcett embarks on the expedition with just his son, opting for a lean approach over several years to find Z. Costin, however, declines to join them. Their travels take a complicated turn as they face hostility from tribes, only to be approached by another group who claim that the Fawcetts' spirits do not fully belong to their realm and offer to assist them in finding their rightful place. In a disconcerting turn of events, the father and son are drugged in a ritual and taken captive.
Months later, a distraught Nina has a meeting with Keltie at the RGS, insisting she has heard rumors that Fawcett and Jack are alive, residing with the indigenous tribes. Despite the RGS's extensive search efforts over the years—including the dispatch of over a hundred searchers—they refuse to conduct another hunt. Keltie, advising her to accept her husband's presumed fate, is met with steadfast resistance. As she departs, Nina produces the very compass that Fawcett had promised to return once he located the lost city, her reflection hints at her next steps leading into what appears to be the depths of the jungle.
