In the late 1800s, when their boat is forced to dock at the village of Puerto Miguel, Mexico, for repairs, Americans Hooker, Fiske and Luke Daly are annoyed that their plans to mine gold in California have been delayed.  Although the three men are strangers to one another, they drink together in a local cantina, where Fiske, a professional gambler, tries to pry personal information out of the taciturn Hooker.  The hot-headed Daly is ready to join in a fight between Mexican Vicente and another man for the attentions of the cantina’s singer, but Fiske warns him to control his temper.

After the incident, a beautiful, desperate American woman named Leah Fuller runs in and begs the locals to help free her husband John, who became trapped in their gold mine during a cave-in.  No one comes forward, even though she offers $1,000 per man, and when she spots the Americans, she approaches them.  Fiske is suspicious about why no one else will take the job, but Leah explains that they merely fear the Apaches who control the territory through which they will be traveling for several days.  Leah doubles the reward and pleads with the men, telling them that a fellow American is dying, and Daly, immediately infatuated with the charismatic Leah, agrees to help.  Hooker and Fiske also agree, as does Vicente, and soon the group is riding along a narrow mountain trail.  Leah reveals to Fiske that she and Fuller were given a map to the remote location by an old priest in Sacramento, and as they continue he is amazed by her stamina.

Hooker and Fiske observe Vicente carefully noting the passing landscape and leaving markers to their trail, and the first night at camp, Fiske taunts Hooker that he, like Daly, will fall under Leah’s spell.  Hooker then prevents Daly from following Leah as she walks away from the camp, and when he himself follows her, he sees her destroying Vicente’s markers.  Leah warns Hooker not to underestimate her, and Hooker retorts that she cares more about her gold mine than her husband.

The following night, the group camps at the burned-out ruins of an old mission, and Hooker finds signs of recent occupation by Apaches.  Hooker cautions his companions that this month, “the moon of the white man,” is one in which the Indians celebrate their victories over white settlements, but Leah shames the men by offering them more money to overcome their fears and continue.  When Leah once again sneaks out of camp to destroy Vicente’s markers, she is followed by Daly, who professes his admiration and violently tries to kiss her.  Leah screams as she fends him off, but then saunters back into camp as if nothing happened.  Hooker orders Daly to go to sleep, and when Daly protests, Hooker reveals that Daly is a cowardly bounty hunter who shoots his prey in the back.  Daly charges at Hooker, but the older man easily outfights him until he begins to weep with humiliation.  Hooker reprimands Leah for inadvertently leading Daly on, then gently tends to Daly, telling him that all Leah cares about is her husband.

The next evening, after they camp, Leah tells Hooker that the abandoned city to which they are headed was once a boom town that was covered by the lava of a huge volcano.  After the explosion, the volcano became sacred to the Apaches, and white miners have been afraid to return.  The following day, the group reaches the massive slide of black lava, and Leah leads them to the mine shaft in which her husband is pinned.  The men succeed in freeing Fuller, and Hooker sets his broken leg.

As Leah tends to him, Fuller bitterly recounts his thoughts during the days he lay waiting for her return, and accuses her of marrying him only so that he could find gold for her.  Leah is stung by Fuller’s perception of her as a hard-hearted fortune-hunter, and when Hooker reveals that they are being watched by Apaches, she offers to stay behind and light fires as a distraction while the men slip away during the night.  Hooker explains that her overwhelming drive for riches has made a coward of her husband, and Fiske accuses her of knowing that none of the men would let her sacrifice herself.  Hooker, who has revealed that he once was a sheriff, organizes the escape party, then slugs Leah and drapes her over his horse.

The group rides as hard as it can, but the still-weak Fuller holds them up, and when they camp, Daly decries his presence as it becomes clear that the Apaches are pursuing them.  Knowing that he is holding them back, Fuller asks Daly for a horse, and before Hooker can stop him, Fuller rides off.  Daly and Hooker are just about to engage in a gun battle when Daly is shot in the back with an arrow, and the rest of the group rides away.  The gruesome sight of Fuller, pierced by arrows and hung upside down on the ruined mission’s cross, stops them, and Leah, believing that he hated her, weeps.  Hooker tries to persuade her that Fuller loved her, and that is why he left their camp rather than endanger her, but she cannot accept his words of comfort.

When they are riding the following day, Apache arrows kill Vicente’s horse, and Vicente is shot to death while hurling challenges at his tormentors.  With only Hooker, Leah and Fiske left, they ride hard until they reach the narrow mountain road, where they hide behind some boulders.  They are able to force the Indians to retreat temporarily, and Hooker then orders Fiske to take Leah to Puerto Miguel while he remains behind to hold off their pursuers.  Fiske refuses to go and, dismising the older man’s assertion that he is a better shot, challenges Hooker to a card game to decide who will stay.  Knowing that Leah and Hooker are in love, Fiske is glad when he wins, and the couple departs.

Upon reaching the meadow, where they know they are safe, Hooker admits to Leah that Fiske is a finer man than he had thought, and that he must rescue him.  Leah acquiesces, and when Hooker returns, the dying Fiske proudly shows him how many Indians he has shot.  Hooker chides Fiske for cheating him in the card game, and Fiske urges him to build a home with Leah.  After Fiske dies, Hooker looks out on the landscape, which the old priest had called “The Garden of Evil,” then catches up with Leah and rides off with her.