Riders of the Purple Sage is an old Western novel and includes traditional themes of Western novels. The book starts off with Lassiter, a lonely gunslinger who is feared throughout the entire West. People believe him to be a cruel and murderous man, as we are led to believe at the beginning of the book. Yet he only uses his weapons for the greater good, such as the mission he is on. Lassiter is looking for his long-lost sister Milly Erne, who was taken from her husband by Mormons and forced to become a wife in their society. She and Lassiter were separated for nearly two decades, but he eventually finds her... dead in the ground with a puny gravestone, on a cattle ranch in Cottonwoods, Utah. Lassiter learns that she gave birth to a child while she was taken away, but was unable to be with her child, dying as a result. He learns this from Jane Withersteen, the owner of the cattle ranch and a close friend of Milly, if not Milly's only friend in this cruel, foreign community.
At this time, Jane is in the spotlight of the town. She owns heaps of wealth, and is by far the wealthiest person in Cottonwoods, after inheriting money from her late father. As a result, many men in the Mormon town want to marry her. Not out of love or the ache of their heart, but rather due to the strong pull of money. Tull, a powerful member of the Mormon Church, desperately wants to marry Jane, but is unable to do so because of her strong bond with another man named Bern Venters. To make himself Jane's only priority, Tull banishes Bern from the town, to a life of solitude and misery, and Bern miserably accepts his horrible fate. In order to prudently finish off the job, Tull sends a group of armed men to beat Bern. Lassiter springs into action in the nick of time and saves Bern from the torture he would've endured.
Unfortunately, Bern has no option but to run. At this point, Riders of the Purple Sage splits into two different stories. In one story, Bern heads towards a place known as Deception Pass, and falls into love with an infamous outlaw named Bess. At the same time in Cottonwoods, Jane and Lassiter are harassed and robbed. Day after day, Tull and the other Mormons steal all of Jane's property, housing, and cattle. When Tull's men come after her, she convinces Lassiter not to fight and the both of them head on horseback towards Deception Pass.
As they run towards Deception Pass, they somehow meet up with Bern and Bess, who have problems of their own. It is revealed to the characters that Bess is the daughter of the late Milly Erne, and Lassiter and Jane know the only way Bern and Bess will survive is on horseback. They give their horses to Bern and Bess, and scramble into a hiding location known as Surprise Valley. The area is full of everything needed to support a family, maybe even an entire civilization, and Surprise Valley's entrance is protected by a gigantic boulder which can be shoved to seal the only entrance to the place. Jane and Lassiter take one last look back upon the land of Utah, as they are pursued, and seal the entrance to Surprise Valley.
