The film opens with a vibrant scene at a Rocky Mountain riverside trading post, teeming with life as Indians, trappers, and traders mingle freely. Here, a young ex-soldier, Jeremiah Johnson, seeks solace away from civilization. He gathers essential supplies, gear, and a pack mule, preparing for a solitary existence as a mountain man.
However, his initial days in the harsh mountains prove challenging, as he soon finds himself lacking both food and survival skills. In a comedic moment, while trying in vain to catch a trout from a swift stream, he is surprised to notice a bemused Indian brave silently watching him. The brave then rides off downstream. Johnson's attempts to start a fire in the snow ultimately lead him to a chilling discovery—a dead mountain man, frozen against a tree. A note reveals that this man, known as Hatchet Jack, was mauled by a grizzly bear, and he leaves behind his prized rifle. Johnson claims the .50-caliber rifle, which becomes instrumental in his success when he bags his first deer.
The next morning, Johnson awakens to find that his horse has perished overnight. Undeterred, he trudges through the deep snow, leading his pack mule. Soon, he encounters an elderly mountain man, Bear Claw, whose bear hunt Johnson inadvertently disrupts. Bear Claw takes it upon himself to impart wisdom about surviving in the wilderness. During one of their elk hunts, they run into a Crow Indian party but, with Johnson's quick thinking, they successfully barter their way out of trouble. Remarkably, the leader of the Crows is the same brave who observed Johnson's earlier mishaps.
After parting ways, Johnson stumbles upon a cabin where a horrific massacre of settlers has occurred, leaving only one child alive, along with his grieving mother who has succumbed to madness. As she attempts to take her own life, Johnson, feeling compassion, takes the traumatized child under his wing. Unable to decipher the boy's name, he chooses to call him Caleb. Their journey leads them to rescue Del Gue, another mountain man, trapped in burning sand by the same Blackfoot raiders responsible for the massacre. Driven by revenge, they track down the raiders, leading to a violent confrontation where Del Gue takes scalps.
This act unexpectedly raises Johnson's status among the locals, as he is honored by the Flathead chief, who sees the captured Blackfoot ponies and scalps, bestowing his daughter upon Johnson. Now with a family, Johnson navigates deeper into the wilderness. Despite the challenges of communication with his mute stepson and wife, he finds joy in this new life. Together they build a cozy cabin near a beautiful mountain stream. However, his happiness is short-lived when a cavalry unit comes looking for his expertise to save a stranded wagon party in peril from the unforgiving cold.
As they venture into the high mountains, they confront a sacred Crow burial ground that complicates their journey. Johnson voices his concerns, suggesting a detour, yet the cavalry dismisses his warnings. Ignoring his instincts, Johnson leads the group through the sacred land, which ultimately draws the ire of the local Crows. When he returns home, he is heartbroken to discover that his wife and stepson have been murdered in brutal retaliation. Consumed with guilt and grief, he lays their bodies to rest and torches their cabin in a fit of despair.
Desperate for revenge, Johnson tracks down the group of Crow warriors responsible for this tragedy. He confronts them without regard for his own safety, resulting in a brutal fight where he manages to kill the entire group, but not without sustaining injuries himself. His relentless pursuit of vengeance makes him a legendary figure, respected and feared among both Indians and settlers. During this tumultuous time, he encounters both Bear Claw and Del Gue once more, who, aware of his struggle, offer their support before parting ways.
After countless battles, a fatigued Johnson spots one last Crow warrior in the distance. Anticipating another fight, he readies his rifle, but to his surprise, the warrior raises his hand in a gesture of respect and peace. At that moment, realization floods over Johnson, and a wide grin breaks across his face—it’s the same brave who once watched him struggle to catch a trout, marking the end of his harrowing journey.
