In Arizona, in 1885, Shefford takes a job on a pack train in order to discover the whereabouts of a secluded gold-filled gorge, "Surprise Valley," where a group of travelers were trapped fifteen years earlier while trying to elude Dyer, an infamous outlaw. The pack train arrives at its destination, a settlement where Dyer and his men keep their "women folk," and Shefford meets a young woman who, unknown to him, is Fay Larkin, one of the Surprise Valley dwellers who has been brought against her will to Dyer's quasi-brothel. Shefford tells her about Surprise Valley and his desire to find and help the people trapped there. Fearing for the safety of the couple who adopted her, Jane Withersteen and Jim Lassiter, Fay tells Shefford that "Fay" is dead and urges him to leave the area. Shefford later saves an Indian girl from the clutches of Willets, one of Dyer's outlaws, and he asks the girl's appreciative brother, Lone Eagle, if he knows about Surprise Valley. When Shefford returns to camp, Willets recognizes him and orders him to be tied up and led to a cliff's edge, but Lone Eagle arrives and threatens that the Navajos will declare war on Dyer's settlement if Shefford is not set free. After Lone Eagle tells Shefford that the young woman whom he met is really Fay Larkin, Shefford promises her that he will rescue Jane and Lassiter. Fay tells him that the valley is marked by a red stone shaped like a rainbow. Shefford goes to the valley, sees Lassiter below and throws him a note stating his promise to return the next day to rescue them. Shefford then returns to the settlement to discover Dyer stabbed. Wearing Dyer's mask and coat, Shefford escapes the area with Fay. Lone Eagle helps Shefford throw ropes down to Jane and Lassiter, as Dyer's men pursue them. Once Jane and Lassiter have been pulled out of the valley, the group heads toward the river in order to escape Dyer's men. They cross a narrow chasm over a fallen tree before disengaging it from the hillside, and thus elude their pursuers. Lone Eagle confesses that he killed Dyer and says that by protecting Fay from Dyer's advances, he was paying Shefford back for saving the woman that he loves. Fay and Shefford embrace.