As a consequence of the Berlin Conference, the Congo is split between Belgium and the United Kingdom. On the verge of bankruptcy due to massive debts incurred for infrastructure projects like a national railway, the Belgian government faces a dire situation. In response, King Leopold II commissions his envoy, Léon Rom, to hunt for the legendary diamonds of Opar. However, disaster strikes when Rom’s expedition is ambushed, leaving him as the sole survivor. A tribal leader named Chief Mbonga proposes to give Rom the sought-after diamonds in exchange for a man who poses a significant threat: Tarzan.
Meanwhile, in London, the man once known as "Tarzan" has distanced himself from the wild and now resides comfortably with his wife, Jane Porter, an American aristocrat. Having adopted his birth name and family legacy, he is now John Clayton III, Lord Greystoke. The past he once lived as Tarzan has turned into folklore among the Victorian audience, yet Lord Greystoke is eager to leave that life behind. When the British Prime Minister invites him to visit Boma and assess the advancements in the Congo under Belgium, he initially declines. However, the American envoy, George Washington Williams, reveals his suspicions about the Belgian enslavement of the Congolese people, convincing John to reconsider for the sake of uncovering the truth.
Though hesitant, John decides to take Jane along, much to her delight, despite his fears of the potential dangers of the expedition—fears stemming from the tragic loss of his parents in the jungle. As they embark on their adventure, the trio arrives in the Congo, where they encounter a tribal village from their past. Jane shares with Williams the notoriety her husband holds among the tribes, including that of Chief Mbonga. She reminisces about their early days in the jungle, when Tarzan heroically protected her from a ferocious ape attack, which drew them together in love.
However, their journey quickly turns perilous. That night, Rom and his mercenaries launch a surprise attack on the village, resulting in the kidnapping of John and Jane alongside the killing of the tribe's leader. In a narrow escape, Williams manages to save John. With the help of the local warriors, they pursuit Rom's steamboat and intercept a Belgian military train transporting enslaved individuals. Here, dramatic revelations unfold, including the construction of a vast railway system intended to place 60% of the Congo within three days of a fort—a project that has yet to see the arrival of its promised troops.
In an unexpected twist, John encounters Akut, his childhood friend now crowned leader of the apes. Their reunion is laced with conflict, as Akut views John's return to civilization as a betrayal. The narrative takes a darker turn when Williams reflects on the grim destiny of Native Americans during the Civil War, positioning their suffering alongside the Congolese plight under Rom and Leopold.
As Jane finds herself at the mercy of Rom aboard the steamboat, she narrowly escapes and seeks refuge only to find herself deep in peril again, navigating an ape nest while witnessing Rom and his men slaughter many of the creatures. As the tension mounts, John reconciles with Akut, uniting forces to confront Rom. A dramatic encounter ensues where Mbonga reveals the complexities of their shared past, with John's previous actions haunting both men deeply.
As Rom withdraws to Boma to finalize his control over the mercenary forces, John, Williams, and Akut unleash a stampede of wildebeests in a dramatic diversion to save Jane. The climax reaches its peak when John confronts Rom, ultimately leading to Rom’s demise at the hands of hungry crocodiles.
Returning to England, Williams presents damning evidence of the slave trade operations in the Congo to the Prime Minister. A year later, John and Jane have made Africa their home, settling into the legacy of Jane's father. With the arrival of their first child, John finds peace and acceptance as he re-embraces his identity among the great apes, stepping back into the shoes of Tarzan.
