In October 1808 fur trapper John Colter set out with another trapper, John Potts, on a trapping expedition. Foolishly, they returned to the Three Forks area in Montana, where they were able to amass almost a ton of furs. However, at Jefferson Fork they were attacked by Blackfoot Indians, who shot and killed Potts and took the furs. Colter was captured and given a chance to live. He was stripped naked and given a 30-second head start to run for his life. He outran all of the Indians except for one. When they were the only two left, Colter turned on his pursuer and in the ensuing fight took the Indian's spear and killed him with it. Colter ran for five miles across a rocky plain between the Jefferson and Madison Forks. Once he reached the Madison River, he dove under a mass of logs and beaver lodges and hid in an air pocket in the icy water until nightfall. He floated six miles downstream and climbed up a sheer cliff. He walked, still without any clothes, the 250 miles to Fort Raymond, where he arrived after 11 days.
Cornel Wilde was careful to try to avoid harm to animals appearing in the film where possible. In the scene where the python and the monitor lizard battle, it became clear that the python was winning and the monitor was in danger. Wilde personally intervened to save the monitor lizard; the lizard bit him on the leg, refusing to let go. Crew members killed it and Wilde had to be evacuated to a hospital for treatment.
The script was originally a true historical incident about a trapper named John Colter being pursued by Blackfoot Indians in Wyoming, but lower shooting costs, tax breaks and material and logistical assistance offered by South Africa convinced Cornel Wilde and the other producers to shoot the film there.
Cornel Wilde said in interviews at the time that scene in which his character turns and is narrowly missed by a spear was an accident that could have been gruesomely real, but the scene worked and was kept.
When Cornel Wilde leaves the fort at the start of the film with the other safari trackers, there are Gert Van Den Bergh, Patrick Mynhardt and Kevin Lee. When the group are attacked, the scene had to be carefully choreographed because many spears were going to be flying through the air. A sturdy cork mat was placed under Lee's safari jacket, with a thin steel line running from his jacket to Cornel Wilde's raised directorial platform, from where he shouted instructions and where the film crew were safe. Wilde had a spear which ran down the line and, once action had commenced, threw the speak which sank safely into Lee's back, and he fell. Other speaks, not directed at specific targets, flew through the air, recklessly. One struck Lee in the foot, piercing through the leather boot of his right foot, and sank into his ankle, narrowly missing his Achilles tendon. When the scene was over, the doctor on standby, had to treat the wound and administer a tetanus injection. But it did put him out of the following scenes. Cornel Wilde was therefore not the only casualty on this movie.