Faith healer Jonas Nightengale and his manager Jane Larson break down in Rustwater, Kansas, a small, rural town suffering from a long drought. Learning they will be stuck there for four days waiting for replacement parts to come in for their truck, Jonas decides to hold revival meetings to help pay for the repairs.
Jonas and his staff use various cons to make it appear as if Jonas has divine knowledge granted by God. Despite his repeated diversions whenever he is asked when the drought will end, the townspeople find Jonas’s shows to be entertaining and hope-inspiring, and eagerly give their money to Jonas as donations.
Local sheriff Will Braverman is skeptical and decides to investigate Jonas' past. He learns that he is in fact Jack Newton, a native of New York City who lived a life of crime in his teen years. Braverman shares this information with the townspeople, who have gathered for another tent revival. Jonas storms off the stage, soon returning to successfully spin Braverman's report, leaving the crowd more energized than ever, much to Braverman's exasperation. Nightengale continues his dubious ministry over the next several days as Jane and Braverman find themselves falling for each other.
Nightengale attempts to begin a relationship with local diner waitress Marva, whose brother Boyd walks with crutches following an auto accident. She explains that doctors could not find anything physically wrong with him and his ailment is psychosomatic. Boyd comes to believe that Jonas can help him walk again. During a meeting, Boyd walks to the crucifix and touches the feet of Jesus. He drops his crutches and begins to walk unassisted. The awed crowd sweeps the stage. After the show, an enraged Jonas rails to Jane that he was conned and that Boyd upstaged him.
After the revival, Jonas enters the empty, darkened tent and mocks the crucifix and Christianity. Boyd walks in while he is talking and says he wants to join him on the road. Jonas agrees to meet him the following morning, implying he can come. Marva arrives and sends Boyd out of the tent. She thanks Jonas, who tells her that he will not be meeting her brother the next morning.
Jonas leaves the tent and sees the crowd that has gathered just outside it — many praying, some sleeping in groups, and others feeding the crowd that has gathered. He begins to understand that Boyd's miracle, and the faith that enabled it, are real after all. He packs a bag and departs alone under the cover of darkness, leaving behind his entire road show and hitching a ride with a truck driver bound for Pensacola, Florida.
When the driver asks if he is in trouble, Jonas replies, "No, no, no, no. No, sir. Maybe for the first time in my life I'm not." As they continue to ride along, the drought, threatening the crop harvest that is the centerpiece of the town's economy, comes to a dramatic end with a miraculous downpour. Jonas laughs silently to himself as he realizes the truth, and the film ends as he rides off into the stormy evening, hanging out the truck window loudly thanking Jesus for the rain.
