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.
In October 1808 Colter set out to trap with another member of Lisa's company, John Potts. Foolishly, they returned to the Three Forks area, where they were able to amass almost a ton of fur. However, at Jefferson Fork they were attacked by Blackfeet who shot and killed Potts and took the furs. Colter was captured and given a chance to live. He was stripped naked and given 30 seconds' headstart to run for his life. Colter outran all of them except for one. When they were the only two left, Colter turned upon him and killed the man with his own spear. Colter ran for five miles across a rocky plain between the Jefferson and Madison Forks. Once he reached the Madison River, he dived under a mass of logs and beaver lodges and hid in an air pocket in the icy water until nightfall. He went 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 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.
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 and the lizard bit him on the leg, refusing to let go. Crewmembers killed the monitor and Wilde had to be evacuated to hospital for treatment.