In New York City, a TV news reporter recounts the details surrounding the mysterious disappearance of a documentary film crew who has gone missing in the jungles of South America for two months. The reporter introduces them as Alan Yates (Gabriel Yorke) the director; Faye Daniels (Francesca Ciardi) his girlfriend and script girl; and friends Jack Anders (Perry Pirkanen) and Mark Tomaso (Luca Giorgio Barbareschi) who work as cameramen. They were last seen departing from the remote Columbian town of Leticia which sits deep in the jungle on the border points between Brazil and Peru, to document the existence of cannibal tribes in the jungle. NYU's noted anthropologist Professor Harold Monroe (Robert Kerman) is assigned the task by the university to find out what happened to the conceited, and over-confident film crew.
In the jungles of South America, a group of Columbian soldiers, led by a balding Negro lieutenant, are on patrol when they stumble upon a group of natives engaged in a spot of cannibalism; eating human remains. The soldiers attack and kill many natives while the rest flee. One fights back by shooting one soldier with a poison dart, hitting him in the arm before he is killed by the other soldiers. Despite the quick medical treatment, the soldier dies soon afterwards. The Columbian Lieutenant then meets with Professor Monroe who arrives by plane at the soldiers outpost on the Amazon. The Lieutenant shows Monroe the captured native which they identify as a Yacumo, for to the soldiers, a Yacumo prisoner is like a ticket into the 'Green Inferno', the inhospitable jungle the South Americans call it. The Yacumo is in possession of a cigarette lighter which Monroe confirms belonged to Faye Daniels. Monroe is assigned a guide, a Venezuelan named Chaco (Ricardo Fuentes), who explains to Monroe that a tattoo borne by the prisoner indicates that he is the son of a shaman, and that the Yacumo are not really cannibals. The conclusion drawn by Chaco is that they were this far out from their village near Leticia where they were probably indulging in a religious ceremony meant to chase evil spirits out of the jungle. White men spirits.
Monroe, Chaco, and another young guide, named Miguel, with their Yacumo prisoner in tow, head off into the jungle, crossing a few streams, encountering the badly decomposed remains of Felipe, the guide of the Yates team near their former campsite. A day or so later, the search party comes across a strange and savage, ritualistic punishment for adultery where a Yacumo man hauls his bound wife to a riverbed where he sexually violates her with a phallic dildo and bludgeons her to death with it. Monroe, Chaco, and Miguel soon reach the Yacumo village where they are met with fear and suspicion. After demonstrating their good intentions of releasing their captive, they are allowed to stay for the night.
The next morning, Monroe, Chaco and Miguel set off on the next stage of their journey, deeper still into the jungle, towards the ultimate goal of the Yates team: meeting the feared Yamamomo tribe, which are also known as the Tree People. A stone-age tribe that has had little or no contact with the outside world and still live in the stone age period. The Yamamomo are constantly at war with a rival tribe called the Shamatari, called the Swamp People. Monroe and his two guides see the Shamatari driving out Yamamomo warriors out of their high up trees by burning fires which the smoke makes the tribesmen fall to the ground. Soon they stumble upon a skirmish between the Shamatari and Yamamomo warriors. Monroe and his two guides intervene by shooting several Shamatari warriors, forcing the rest to flee and establishing themselves as allies to the Yamamomo. The Yamamomo tribesmen permit Monroe and his guides to stay, but treat them with fear and suspicion as did the Yacumo which confirms to Chaco that the Yates team did something to stir up trouble. Despite the language barrier, Monroe becomes determined to find out from the Yamamomo what happened to the Yates team.
The following day, Monroe gains the trust of the Yamamomo by stripping off his clothes and bathing in a nearby stream. A group of carefree Yamamomo women arrive to bath with him and playfully splash around in the water, while Chaco and Miguel watch from the riverbank. After a few minutes, the Yamamomo women lead Monroe and his guides away from the river, into the jungle, and to a clearing where they are introduced to a bizarre totemic shrine: the remains of the Yates team, with their four skulls arranged in a ghoulish manner confirming that the film crew is dead and that the Yamamomo have been trying to exercise the jungle of their spirits.
Seeing the crew's film cameras and the silver canisters of film untouched by the Yamamomo custom, Monroe decides to use another psychological manipulation to gain access to the film reels. Monroe manages to manipulate the Yamamomo chief by showing him a cassette recorder which has the sounds of the Yacumo and other tribes chanting, The chief accepts the recorder as a gift and allows Monroe, Chaco and Miguel to partake in a ritualistic cannibal feast by eating part of the dead body of a Yamamomo criminal recently executed. The Yamamomo chief then allows Monroe to leave with the film canisters.
Several weeks later, Monroe arrives back in New York where he gives a TV interview about his experiences in the jungle and about the death of the Yates team and that the film reels recovered may prove the answer of what brought on the deaths of the Yates crew. Monroe meets with three NYU board executives, two men and a woman, who want him to review the film in preparation for a TV documentary to be shown as Yates final work in the Amazon.
Monroe is shown the Yates footage at office where the female executive introduces Monroe to Yates style of filmmaking by showing him extracts from a film he made in Africa a year earlier entitled 'The Last Road to Hell' showing archive films of firing-squad executions. At this point, Monroe sits down in the editing room to watch the raw footage of the last days of the Yates party. The Yates team is shown from their hotel in Leticia to traveling by plane several miles away to a point to hike into the Green Inferno to make contact with the legendary cannibal tribes. Monroe watches the arrogant and superficial film crew of Yates, Anders, Faye, Mark, and their guide Felipe hike into the jungle, traversing the same streams Monroe and his group crossed months later, and them bragging to the camera about the natural beauty of the jungle, while they commit various and unforgivable atrocious against various wildlife. In one repugnant scene, Felipe and Anders catch a large river turtle, they haul it ashore at their camp, and proceed to hack it apart before cooking and eating it while the camera is still rolling. Later, they shoot at various birds and small mammals in the jungle for no reason, but then Felipe screams out when a snake bites his foot. Anders and Mark, in desperation, hack off Felipe's leg in a vain bid to save him, but Felipe goes into shock and dies thus revealing how he died. Yates tells his team that they must continue on.
Taking a break from watching the footage, Monroe then embarks on intervewing some of the Yates team family members, some of whom tell Monroe little about the death of their relatives, and a few who scorn them for the trouble they would stir up. As Monroe begins to realize with growing horror about what really happened to the Yates team months earlier, he proceeds to view the rest of the film:
After the death of their guide, Yates, Faye, Anders, and Mark head further into the jungle and finally make contact with warriors of the Yacumo tribe. For no clear reason, they shoot at the natives, and then upon entering their village, proceed to humiliate the natives by killing their livestock, and proceed to burn their village to the ground, in which they intend to use the killing to stage a mock up of a Yamamomo raid on their village. Several natives flee, while a few others are killed or burned alive by the sadistic Yates team. Then Yates, aroused by his murderous actions, has sex with Faye in full view of the frightened and bewildered natives, while Andres secretly films it. In the films most gratuitous scene, the team happens upon a ritualistic forced abortion and the clubbing death of the mother-to-be.
Monroe tells the female executive, who also viewed the footage, against broadcasting it as he attempts to persuade the executives to abandon their plans to air the footage. But his request falls on deaf ears as the executives want callous sensationalism which the public needs to thrive on with the brutal footage which will bring ratings to their company.
The next day, Monroe's anxiety has turned into anger when he meets again with the three executives and tells them that he has viewed the final reel and says that it is "offensive, dishonest, and inhuman". He leads the executives into a screening room to view the final reel of the film:
In the final reel, Yates speaks to the camera where it is days after their massacre of the Yacumo tribe and they are in a different part of the jungle for the vegetation is different and there is no sign of the feared Yamamomo tribe. Suddenly, the group happens upon a young woman, a Yamamomo woman. Triumphantly and inexplicably, the men attack her, taking turns raping her while rolling in the mud of a nearby clearing, while one man holds her down, the other rapes her, and the third gleefully films the action. Faye tries to protest, but is held back by Anders. As Yates takes his turn to rape the terrified young woman, a Yamamomo warrior crouches in the grass yards away, watching the whole atrocity the savage film crew is doing. The film cuts to a riverside clearing where the mud covered Yates team find a dead Yamamomo woman vertically impaled to the ground with a 12-foot pole exiting out of her mouth. Yates looks overjoyed and is told to look concerned for the camera. Yates talks about the profound disrespect the Yamamomos have for their women and that the woman was killed as a form of sexual punishment. It is not clear whether the woman was killed by her own people for being sexualy violated by the Yates team, or if Yates and his group himself killed the woman themselves to rig up this latest spectacular footage for their people back home. As a result of their actions, the vengeful Yamamomo emerge for a final 10-minute violent confrontation. In the jungle, the group is surrounded by dozens of Yamamomo warriors throwing spears and arrows at them. Anders fights them by rashly shooting at several of them, while Yates (deranged to the last) continues filming, saying the footage is beautiful. Anders is felled by a spear, as Yates continues filming. Anders' dead body is dragged away by the Yamamomo where they castrate, behead, and hack him apart before cooking his remains and eating them. Faye is the next as she is grabbed by several Yamamomo warriors where she is stripped, gang raped, and is also beheaded and mutilated by the vengeful warriors. But it isn't long before Yates and Mark pay the price for their own indiscretion when the warriors spot Yates still filming them, and attack. When Yates is attacked, his keeps his camera rolling as he drops the camera and falls to the ground capturing his own horrifying death in close-up. In a fitting touch of irony, the last thing which his dying eyes see is the same camera lens which he and his crew used to capture the images of death and mayhem...... which in fact they were all responsible for.
Back in the screening room, Monroe looks at the three executives, who look on with silent shock and disgust. Finally agreeing with Monroe's assessment at last, one of them orders that the film be destroyed and not shown. As Monroe walks out of the building and into the hostile concrete jungle of New York, he looks around at the people on the streets and mutters, "I wonder who the real cannibals are".