Set in the White Mountains of Arizona, the novel follows Milt Dale, a solitary hunter, and Helen Rayner, a young woman from Missouri who travels west to inherit her dying uncle's ranch. Their lives converge when Dale overhears a conspiracy that threatens Helen's safety and future.
One September evening, Dale hides in the loft of an abandoned cabin when five riders arrive: Snake Anson's gang of sheep-thieves. Beasley, a powerful local rancher, meets the gang and reveals that his former partner, the aging Al Auchincloss, is dying and has sent for his niece Helen to inherit his property. Beasley proposes that Anson's gang intercept Helen at the railroad town of Magdalena, kidnap her, and make her disappear. With Auchincloss dead and Helen gone, Beasley plans to claim the property by asserting old debts. He offers Anson 10,000 sheep or their equivalent in gold. Dale resolves to prevent the abduction.
Dale travels to the village of Pine and rides to the Auchincloss ranch to warn the old man, but Auchincloss refuses to listen and orders Dale off the property. Dale turns to his four Mormon friends, the Beeman brothers: John, Roy, Joe, and Hal. They have their own grievances against Beasley, who ruined their father, and they suspect him of profiting from Anson's sheep-stealing. The brothers pledge to help.
Meanwhile, Helen, a 20-year-old schoolteacher, leaves St. Joseph, Missouri, with her 16-year-old sister, Bo Rayner. On the train, they discover that Harve Riggs, a boastful, self-styled gunfighter who has long pursued Helen, has followed them. At a stop in Las Vegas, New Mexico, a handsome young cowboy flirts with Bo from the platform. Bo is immediately smitten.
At Magdalena, Dale meets the girls. When Riggs intervenes possessively, Dale disarms him with a swift blow. He leads the sisters to a stagecoach and explains that the regular stage would be ambushed. They take a different route, riding cross-country through freezing darkness. Dale gives Helen his finest horse, a black named Ranger. The three-day journey to Dale's hidden mountain camp is grueling, marked by cold, exhaustion, and treacherous terrain.
They descend at last into a breathtaking valley enclosed by towering cliffs, with a waterfall pouring from a notch in the mountain wall. Bo names it Paradise Park. Dale's camp lies among huge rocks and pines near a spring. His pets include Tom, a tame cougar; Bud, a bear cub; and Pedro, a loyal hound.
During their extended stay, Dale teaches the sisters to ride, saddle horses, and shoot. Bo embraces the wild life with abandon, while Helen is more contemplative but equally transformed. Dale engages Helen in conversations about nature, explaining how predators maintain the health of prey populations and how all life exists in a balance of strife and survival. He argues that nature's religion is simply to live, grow, and reproduce. Helen challenges him with her belief in God and the soul. Dale tells her that her education has separated her from her instincts and that the West will awaken her to realities she has never known: mortal danger, the desire to kill, and falling madly in love. Bo teases Helen that she will fall in love with the hunter. Helen denies it, but Bo declares there is nothing else in the world but love.
A grizzly bear hunt proves Dale right. When the bear is tracked on horseback, Helen's horse bolts. She is thrown, remounts, and rides in furious pursuit down a mountainside, crashing through timber. She catches up to Dale and Bo, bloody and disheveled but alive with fierce energy. Dale asks if she has woken up, and she admits he was right: She did not know herself. Later, on a nighttime expedition to shoot the grizzly, her horse throws her again. As Dale lifts her from the ground, Helen realizes she has fallen in love with him.
Weeks later, Auchincloss arrives at Paradise Park with his men. Among them is a new cowboy named Carmichael, the same young man who flirted with Bo at the Las Vegas station. Auchincloss thanks Dale and privately urges him to pursue Helen, but Dale refuses, fearing what he might discover about himself. Helen bids Dale farewell; he gives her Ranger and Pedro, promising to come to Pine in the spring. As she rides away, Helen realizes that Dale has fallen in love with her without knowing it.
Alone through the winter, Dale finds his wilderness empty. Haunted by Helen, he comes to believe his solitary life is a form of selfish withdrawal. When heavy snow seals the passes, he confesses aloud that he loves Helen Rayner but believes she could never care for a man of the forest.
At the ranch, Helen manages the estate as Auchincloss's health fails. Carmichael becomes her indispensable ally and falls deeply in love with Bo, though their relationship is volatile. Helen learns from Carmichael that foreman Jeff Mulvey plans to desert to Beasley's side when Auchincloss dies. Auchincloss dies in mid-February, leaving everything to Helen. Beasley proposes marriage to settle his claims; Helen refuses and reveals that Dale overheard his conspiracy. Roy Beeman confronts Beasley in Turner's saloon, and Riggs shoots Roy from behind. Roy survives but is seriously wounded. Bo tells Carmichael, who beats and humiliates Riggs, driving him out of town.
Bo, riding alone despite warnings, is chased and roped by Riggs, who delivers her to Snake Anson's camp in the mountains. Anson is furious that Riggs brought the wrong sister. Jim Wilson, a Texan in Anson's gang, grows protective of Bo, sheltering her each night.
John Beeman brings Dale the news. Dale arrives in Pine, and Helen throws her arms around his neck. He promises to bring Bo back and rides out with Tom the cougar. He tracks the gang to a remote glen, where he encounters Wilson, who agrees to help and reveals he killed Riggs for striking Bo. They devise a plan: Wilson tells the outlaws Bo has gone insane and died, while Dale uses Tom to terrorize the camp. The cougar's screams stampede the horses and shatter the outlaws' nerves. Anson's panicked horse crushes him. Wilson carries Bo to Dale, and she thanks Wilson before he walks into the darkness.
Dale brings Bo safely to Pine. In his absence, Beasley's henchmen have seized Helen and thrown her off the ranch. Carmichael gallops in, overjoyed to find Bo safe but wild with fury. Helen intercepts Dale as he follows Carmichael toward the village. She tells him she knows he loves her and declares her own love. Dale admits his feelings but insists he must confront Beasley. Helen tells him that if he kills Beasley, she will never marry him. Before the standoff resolves, gunshots ring out from Turner's saloon: Carmichael has found and killed Mulvey and one of Beasley's henchmen. Dale carries the fainting Helen away.
Carmichael rides to the ranch, where Beasley's remaining men refuse to fight. He finds Beasley in the dining room and taunts him, giving him every chance to draw. Beasley offers money, then the ranch, then full surrender. Carmichael refuses. When Beasley grabs for his gun, Carmichael kills him.
Two months later, Helen and Dale run the ranch prosperously. Carmichael returns from an absence leading a blue roan mustang as a gift for Bo, with the condition she can ride it only as Mrs. Tom Carmichael. Bo calls his bluff and tells him to find a preacher. Roy reveals he is ordained, and Bo and Carmichael marry that day.
Dale asks Helen to return to Paradise Park for their wedding. Under the pines at sunset, Roy performs the ceremony. His prayer asks God to bless their children with Dale's strength and Helen's love, "which must be the softenin' an' the salvation of the hard West" (175). Tom the cougar's mournful cry echoes from the dark slope. Dale and Helen sit by the campfire as he vows to homestead Paradise Park, and Helen whispers the name as the wilderness enfolds them.