During the Mexican–American War, Second Lieutenant John Boyd, who is fighting in the United States Army, finds his courage failing him in battle and plays dead as his unit is massacred. His body, along with the other dead, is put in a cart and hauled behind Mexican lines. However, in a moment of bravery, Boyd seizes the chance to capture the Mexican command post. His heroism earns him a captain's promotion, but when General Slauson learns of the cowardice through which victory was achieved, he posts Boyd into exile at Fort Spencer, a remote military outpost high in the Sierra Nevada commanded by the weary but genial Colonel Hart, and staffed by a motley array of misfits: the pious Private Toffler, the drug-addicted Private Cleaves, the drunken Major Knox and the ferocious Private Reich, in addition to the Native American scout George and his sister Martha.
Shortly after Boyd joins the garrison, a frostbitten stranger named Colqhoun arrives telling a hellish tale about how his wagon train became lost in the mountain because a Colonel Ives had promised the party a shorter route to the Pacific Ocean. Instead he led them on a more circuitous route resulting in the party getting trapped by snow for three months. Wracked by starvation, Colqhoun and his fellow travellers were reduced to cannibalism, while he alleges that Ives resorted to murder. A rescue party is assembled to retrieve any survivors and capture Ives. George warns those who are leaving about the Wendigo myth: anyone who consumes the flesh of their enemies takes their strength but becomes a demon cursed by an insatiable hunger for more human flesh.
When the soldiers reach Ives' cave, Boyd and Reich investigate. They discover the bloody remains of five skeletons, and realise that Colqhoun is Ives and he murdered everyone. Colqhoun's plan is now to kill and eat the soldiers. Colqhoun quickly kills George, Toffler and Colonel Hart, and, after a brief struggle, Reich. Boyd escapes the massacre by jumping off a cliff but breaks his leg. He hides in a pit next to the body of Reich whom he eventually eats to stay alive.
When a delirious and severely traumatized Boyd finally limps back to the fort, he returns to find it has been reinforced by General Slauson and a detachment of cavalry. Cleaves and Martha (who were on a supply mission and had not met Colqhoun) do not believe his wild tale, while the hung-over Knox cannot recall and refuses to back Boyd up. A second expedition to the cave finds no bodies or any trace of Colqhoun. A temporary commander is assigned to the fort but to Boyd's horror, it is Colqhoun, who is now calling himself Colonel Ives again. The men still refuse to believe Boyd because Colqhoun bears no sign of the wounds inflicted on him during the fight at the cave.
Secretly, Colqhoun tells Boyd that he used to suffer from tuberculosis but when a Native scout told him the Wendigo myth, then he "just had to try" by murdering him and eating his flesh, a process that cured his disease. Colqhoun now plans to use the fort as a base to cannibalize passing travellers because, like the notion of Manifest Destiny, the migrants had a calling just like himself. Boyd is soon suspected of murder after Cleaves is mysteriously killed. While chained up, he watches helplessly as Knox is murdered by Colqhoun's unexpected ally: Colonel Hart, back from the dead after the massacre. Colqhoun had saved Hart by feeding him his own men in order to gain his assistance. But like Colqhoun, he is now hopelessly addicted to human flesh. Colqhoun mortally wounds Boyd, forcing him to make a choice: eat or die.
Eventually, Boyd gives in and eats a stew made from Knox. However, rather than join the two men in their conspiracy to convert General Slauson, Boyd convinces Hart to free him so he can kill Colqhoun. Hart does so, but also asks to be killed because he no longer wants to live as a cannibal. Boyd agrees to this. Boyd and Colqhoun fight, inflicting grievous wounds on each other, as their recuperative powers sustain them. Eventually, Boyd forces Colqhoun into a large bear trap that pins them both together. Colqhoun taunts Boyd by telling him that he will eat him, but soon dies. General Slauson returns, and while his aide looks around the dilapidated fort, the general tastes the meat stew left simmering on the fire. Martha sees the deceased Colqhoun and the dying Boyd together, closes the door, and walks away. Boyd does not eat Colqhoun and dies.
