Lenzi was probably trying to imitate the content of notorious Mondo cinema, which had gained considerable Grindhouse popularity since Gualtiero Jacopetti and Paolo Cavara made Mondo Cane in 1962, even though this film is fictional. Like Man from Deep River, Mondo films often focus on exotic customs and locations, graphic violence, and animal cruelty.
The story focuses on a British photographer, John Bradley, who is sent into the Thai rain forest to take wildlife photographs. While on assignment, a tribe native to the area takes him captive.
Bradley starts in Bangkok, taking photos and seeing the sights, until he arrives at a boxing match with a date. His date grows increasingly bored and disgruntled by Bradley's refusal to leave, until she finally walks out on him, which doesn't bother him in the slightest. An unidentified man sees her leave, and presumably upset over the disrespect shown towards the young woman, he follows Bradley to a bar where he confronts him with a knife. After a brief struggle, Bradley manages to turn the weapon against the man and kills him. Even though he killed in self-defense, Bradley immediately flees the scene.
The next day, John begins his trip deep into the rain forest. He rents a canoe and a guide to take him down a nearby river. Still fearing that he'll be captured by the authorities, he pays off the man to not mention their encounter. After rowing a ways and taking several wildlife photos, Bradley's guide, Tuan, mentions his concerns about traveling so far down river, which he reports as being dangerous. John agrees to head back after one more day of traveling. John falls asleep, and when he awakes, he finds Tuan dead with an arrow in his throat. Before he has any chance to escape, a native tribe captures him in a net and carries him to their village. The chief, Luhanà, is told that the group has captured a large fish-man. At the village, Bradley is hung in the net from a high pole, where a group of young children hit him with bamboo stalks. While hanging, Bradley witnesses the execution of two war criminals by his captive tribe. The tribe is at war with another, even more primitive tribe of cannibals, the Kuru. Two of the cannibals have their tongues cut off in the village center. Bradley reacts with disgust, labeling the tribe as murderers.
Still in the net and hanging for hours, John notices that he has attracted the attention of Marayå, the beautiful and naked daughter of the chief who takes an immediate fascination with the stranger. She convinces her father that John is not a fish-man, just a man. Luhanà agrees to release Bradley as Marayå's slave. He is forced to stay locked in a shack for hours, where Taima, Marayå's governess, introduces herself. She is a missionary child and can speak English, and tells Bradley that soon he will be released, as Marayå will be married to Karen in ten days. Luhanà interrupts the two and unties Bradley because it is the day of the Feast of the Sun. During the feast, a helicopter flies overhead. Bradley tries to be rescued, but he is subdued by other warriors, who nearly kill him. Marayå intervenes, however, protecting her property. The helicopter gives John hope, and he plans escape, which Taima agrees to eventually help him with.
A month passes, as Bradley grows even more tense. During one day of labor, a building accident kills a young man. Bradley watches the funeral ceremonies and is again shocked by the actions of the natives. During the ceremony, Taima tells Bradley that now is his time to escape. He does, but Karen and a group of warriors chase after him. They corner him at a waterfall, where Bradley kills Karen. Again a helicopter flies by, and again John goes unnoticed. After Karen's death, the tribe decides to incorporate Bradley as one of them. He faces various rituals and tortures until he is finally released and accepted as a warrior, and he uses his knowledge of modern technology and medicine to help the tribe, but, as a result, becomes an enemy of the tribe's witch doctor. During this time, he and Marayå begin to become fond of each other, until Marayå must choose a new fiancé. Of the tribe's warriors, Marayå chooses John, and the two are married. After the wedding, the two run into the wilderness where Bradley gets naked and has sex with Marayå. This ends up getting Marayå pregnant. During the conception, however, a black butterfly flew over the two lovers, a foreboding of ill fate.
It is now six months after Bradley has been captured, and he has finally accepts his new life with Marayå. However, this is also when the cannibals decide to strike. Two teenagers, a boy and a girl, are ambushed outside of the village by the Kuru. The girl is killed and the boy mortally wounded, but he still is able to inform the others of the attack before he dies. John joins other warriors to eliminate the attack party, and they arrive to see the Kuru party consuming the young woman. The group attacks the cannibals, with John participating in activities he earlier condemned. When Bradley returns, however, he learns that Marayå has fallen ill from the pregnancy and has been stricken blind. John believes the only way to save her is to take her back to civilization for modern medicinal treatment. Taima helps the two escape, but she is caught in doing so has her hand cut off as punishment. Bradley and Marayå are captured and forced to return.
Upon their return, Marayå goes into labor. Again Bradley rejects the witch doctor, sending him away from the ailing Marayå. At this time, the Kuru return to attack the village. They set fire to many huts before John and the other warriors are able to react. In the ensuing and graphic battle, John takes Marayå to safety until the cannibals are fought back and withdraw. As John tries to comfort Marayå's pain, he points out a black butterfly overhead. Marayå then reveals the significance of the butterfly: death. Marayå finally gives birth, and dies shortly after. John wanders aimlessly through the jungle upon his wife's death, only sadly recollecting memories of her. Again a helicopter flies overhead, and after a moment of contemplation, he takes cover with the rest of the tribe, deciding to stay with them, probably for life, to help them rebuild and live against the Kuru and the elements.
Though the "cannibal boom" of the 1970s and 1980s did not start until Ruggero Deodato released his film Last Cannibal World in 1977, Man from Deep River is seen as either the inspiration or the start of the cannibal genre, as the combination of the rain forest setting and onscreen cannibalism was not seen until its release (ironically, director Umberto Lenzi would admit that cannibalism was not intended to be the central theme). When released in America, it would prove successful on Time Square's 42nd Street under the title of Sacrifice!, offering the opportunity for similar films to enjoy that same success (which was ultimately the case). Lenzi was even given the chance to direct Last Cannibal World, but the producers chose Ruggero Deodato when they refused to match Lenzi's price. He would, however, make a follow up in 1980 with his film Eaten Alive! (which even featured the Grindhouse theaters of 42nd Street that had made Man from Deep River famous) and his most famous work,Cannibal Ferox.
Other than being the first cannibal film, Man from Deep River is also notorious for several scenes of extreme violence and gore, which is standard for its genre. Though several scenes of torture and cruelty are present, its inclusion of several on-screen slayings of animals tends to land the film in hot water with censors all over the world.
A large amount of the film's notoriety comes from its inclusion in the UK's list of video nasties, films that the Department of Public Prosecutions (DPP) deemed obscene. Though it was rejected for cinema release and certification by the BBFC in 1975, it was still able to make it to a video release under the title Deep River Savages. When the DPP compiled the "video nasties" in 1983, Deep River Savages made its way onto the list. In 1984, the Video Recordings Act was instated by the British Government, and Deep River Savages was banned from the UK in its entirety (largely due to the real animal killings). In 2003, Deep River Savages was again brought before the BBFC; it was passed with a certificate of 18 after being cut by nearly four minutes to remove all animal cruelty present, and was again subject to three minutes of similar cuts when resubmitted in 2016. Ironically, despite all the controversy surrounding the film's UK release, Man from Deep River was passed with a simple R rating by the MPAA.