Set in an indeterminate year of the 1820s, the opening scene shows Hugh Glass (Leonardo DiCaprio) speaking to his son Hawk (Forrest Goodluck) in an Indian language (Pawnee), telling him that even though he is scared and wants his trouble to be over, he must fight as long as he can grab a breath. As we hear Glass' voice, we see images of Glass with his Pawnee wife and son, his home being set on fire, and him holding his wife in his arms.
Glass and Hawk are walking through a river with other men from their hunting party as they stalk elk. They are camped by a river in rural Missouri with other fur-trappers. They are led by their captain, Andrew Henry (Domhnall Gleeson). A naked missing man from their party walks into the settlement and collapses forward with an arrow in his back. Another man is shot in the neck with an arrow and falls into the campfire. The hunting party is attacked by a tribe of local natives who are Arikara Indians, also known as 'Ree'. The men fire back with their rifles. Glass is attacked and nearly choked to death, but one of his men rescues him. A few men gather their furs and materials toward a boat to make their escape. The Arikaras themselves are looking for the chief's daughter, Powaqa, stolen by unknown white men.
The men abandon their boat and start to hike overland to Fort Kiowa. Hawk is antagonized by John Fitzgerald (Tom Hardy) for his race (Hawk is half-Indian on his mother's side), but Glass quickly defends his son and tells him not to retaliate against Fitzgerald.
While hunting in the woods, Glass comes across a grizzly bear and her cubs, and he is quickly attacked by the larger bear. The bear claws and bites Glass, throwing him around as Glass tries to defend himself. He manages to shoot the bear, but it doesn't kill her. She attacks again and Glass gets his knife, stabbing it several times as they both tumble down a hill. Glass lands in a gully and the dead bear lands on top of him. The men later find Glass and try to tend to his deep wounds.
Meanwhile, the Arikara warriors continue their search for Powaqa. They come across French fur traders and trade the riverside furs for five horses.
The men in the hunting party carry Glass on a makeshift stretcher, but he only slows them down. They attempt to carry him up a hill, only for him to slide and bring the other men down. Fitzgerald suggests they need to just kill Glass and put him out of his misery. Henry covers Glass' eyes and almost shoots him in the head, but he cannot bring himself to do it. Henry offers payment of $75 to anyone who will stay behind with Glass. Hawk and Jim Bridger (Will Poulter) volunteer, though Fitzgerald points out that they and Glass will be likely to die. Henry raises the payment offer to $100 so that even Fitzgerald decides to stay with them until Glass expires.
Glass is still in great pain and continues to have visions of his wife and the day his home was burnt down.
Fitzgerald gets a moment alone with Glass and tries to convince him to let him put him out of his misery so that no one else is slowed down or left waiting to die, including Hawk. As Glass is unable to talk, Fitzgerald suggests Glass should blink if he agrees, knowing that Glass would eventually have to blink, with or without intention to agree to Fitzgerald's offer. Glass holds his eyes open for a long time before closing them, instead of blinking. Fitzgerald intentionally interprets this as blinking and starts to smother Glass. Hawk shows up, seeing Fitzgerald smothering his father. Hawk starts to call Bridger for help, leading to a struggle with Fitzgerald in which the man stabs Hawk in the abdomen, then letting him bleed out as Glass watches helplessly. Fitzgerald gets rid of Hawk's body and tells Bridger he doesn't know where he went.
Later that night, Fitzgerald urges Bridger to move on with him, claiming to have seen Ree Indians by the creek. Already having dug a grave for Glass, Fitzgerald forcefully drags him into the hole and partially buries him alive under a pile of dirt as Bridger reluctantly lets him do so.
Meanwhile, Henry and the rest of the hunting party have a difficult march by as they head towards Fort Kiowa. As Fitzgerald and Bridger head to meet them, Bridger realizes Fitzgerald was lying about having seen the Ree by the creek. He turns his rifle on Fitzgerald, who takes it from Bridger and turns it on him. He pulls the trigger, but the the unloaded rifle clicks on an empty chamber. They continue to move.
Glass awakens and weakly struggles to rise from out of the dirt. He starts crawling his way through the woods to find food and warmth. He finds Hawk's body freezing up from the cold. Glass vows to stay by his son's side. He finds a thick bear pelt to take with him to keep warm. As he continues to move through the woods, he feeds off of roots and old bone marrow. He attempts to build a fire for added warmth and uses some of his leftover gunpowder to seal the wound in his throat. Nearby, the Indians are getting closer, so Glass rides down the rapids to escape them.
Fitzgerald and Bridger are still walking. They come across a burnt-down settlement with bodies sprawled across the ground. One woman emerges from her burnt hut and sees the men. Bridger quietly leaves some food for her.
Glass is getting colder and hungrier. He walks into the river and eats a live fish. He walks up a hill and sees a Pawnee Indian feeding off the carcass of a bison. Glass approaches him cautiously and gestures for food. The Indian throws him an organ, which Glass eats ravenously. In the morning, the Indian observes the bear wounds on Glass' body, which are starting to rot. Glass says his men left him for dead and killed his son. The Indian states that his own family were killed by a rival Sioux tribe. He is seeking out more Pawnee.
Fitzgerald and Bridger reach the outpost and rejoin their party. Fitzgerald tells Henry that they couldn't save Glass or Hawk, and he collects his payment. Bridger remains quiet but is upset and refuses a bonus pay.
Glass and the Indian move forward. They spend the evening sitting and catching snow in their mouths, the first time Glass has looked peaceful in a while. The Indian gathers materials for a quick sweat lodge and places a feverish Glass inside. Glass starts hearing his wife's voice, and then sees himself walking toward Hawk before they embrace in an old church. The Pawnee performs a healing ritual for Glass' wounds.
When Glass wakes up, the Pawnee is gone. A sort time later he sees that the Indian has been hanged by the French fur trappers. He infiltrates their camp and witnesses one of the men raping a woman. It is Powaqa (Melaw Nakehk'o). Glass holds the rapist at gunpoint and frees Powaqa. Glass then takes a horse, letting the other horses loose. He rides to a spot in the woods where he builds himself a fire.
In the morning, the tribe searching for Powaqa starts to attack. Glass holds them off with his rifle before he mounts his horse and rides away. The tribe follows him on their horses up to a cliff where Glass and his horse fall over the edge. The horse dies, and Glass is injured again. As the night falls and the cold intensifies, he cuts the horse open, removes its organs, and bunks inside its carcass for warmth. When he wakes up, he gets out of the carcass and moves to a snow cave. In there, he carves "Fitzgerald killed my son."
Some time later at the outpost, one of the French hunters arrives with Glass' canteen, which Bridger left on the dirt pile after Fitzgerald buried him. Thinking he took it from Hawk, Henry leads a search party through the woods. There, they find Glass, limping towards them. They bring him back to the outpost. Henry finds that Fitzgerald is gone. The French hunter tells him that he heard Fitzgerald was headed to Texas. What's more is that Fitzgerald cleaned out the party's payroll safe. Henry then confronts Bridger with his rifle and beats the young man to the ground and puts him in the stockade.
Glass vouches for Bridger's innocence to Henry, stating that he was only following orders. He also tells Henry how he saw Fitzgerald kill his son. Hearing that Fitzgerald is heading for Texas, Glass requests that he seek the man out himself. Henry reluctantly agrees to have Glass join him in the hunt.
The two encounter Fitzgerald and split up to get him from opposite sides. Henry finds Fitzgerald first and plans to bring him back to be tried for murder. The two men draw their guns on each other, with Fitzgerald killing Henry. Glass finds Henry's body and puts him back on top of his horse using a branch as a prop. They ride on in view of Fitzgerald, who fires his rifle from a distance. He thinks he's killed Glass, but he just shot Henry's body. Glass gets a shot off and wounds Fitzgerald who runs while Glass pursues. They corner each other in the woods, and Fitzgerald shoots at Glass. Fitzgerald runs down by the creek where Glass finds him and they begin to fight. Fitzgerald nearly stabs Glass, but Glass turns the knife on him. Fitzgerald impales Glass' hand, but Glass overpowers him and nearly finishes him off until Fitzgerald states that killing him won't bring his boy back. On the other side of the creek, Glass sees the Indians that have been pursuing him, now with Powaqa. He decides that revenge is in God's hands, so he pushes Fitzgerald into the water and lets him float over to the Indians. The chief grabs Fitzgerald and kills him with his knife. They spare Glass since Powaqa tells them that Glass freed her.
Although he has gotten his revenge, Glass is alone once again, wandering through the cold land. He falls on his knees and sees a vision of his wife once again walking into the light. The final shot is of Glass' eyes filling up with tears.