In 1939, drifter Stan Carlisle obtains a job at a carnival and watches a geek show in which a deranged man eats a live chicken. He begins working with the clairvoyant act starring "Madame Zeena" and her alcoholic husband, Pete. They use coded language and cold reading tricks, which Pete keeps in a secret book. Pete teaches tricks to Stan, warning him against using them to pretend to speak to the dead. Stan also learns that the owner, Clem, hires troubled alcoholics to be his "geeks," then gives them opium-laced alcohol so they stay. Stan becomes attracted to fellow performer Molly and asks her to leave the carnival with him, but she refuses. One night, Pete asks Stan for alcohol and is later found dead. Stan saves the carnival using his cold reading skills to convince the sheriff not to close it. Molly agrees to leave the carnival with Stan, to whom Zeena has given Pete's book.

Two years later, Stan has successfully reinvented himself with a psychic act for the wealthy elite of Buffalo, with Molly as his assistant. During a performance, their act is interrupted by psychologist Dr. Lilith Ritter, who attempts to expose the rigged act. Stan bests and humiliates Ritter with his cold reading skills. Stan also cold reads Judge Kimball. The judge offers to pay Stan for a private consultation to help him and his wife communicate with their dead son. Despite Molly's objections, Stan agrees.

Ritter arranges a meeting with Stan to exchange information: If he is truthful about his background, she will give him information on Kimball. During their session, she learns Stan may have killed Pete and also that his alcoholic father neglected him as a child. With Ritter's information, Stan performs a successful reading for Kimball. He offers to split the profits with Ritter but she refuses, agreeing to keep the money in her care to hide it from Molly. Stan informs Ritter that Kimball has offered to introduce him to Ezra Grindle, a former patient of Ritter's. She feeds Stan information about Grindle's past, namely how he forced an abortion on a young woman named Dorrie. Stan and Ritter begin a romantic relationship, and he begins drinking.

Stan continues to hold sessions with Grindle, who demands that Stan make Dorrie's spirit materialize in a séance. Stan agrees and plans to have Molly pose as Dorrie from afar. Meanwhile, Judge Kimball's wife murders her husband and commits suicide to be reunited with their son, whom Stan said was waiting for them. Molly initially refuses but relents with the promise of being able to leave Stan if she cooperates. During the act, Grindle realizes it is a scam; he hits Molly and threatens Stan. Stan beats Grindle to death and runs over Grindle's bodyguard with his car. Horrified, Molly leaves.

Stan arrives at Ritter's office to obtain his money, but she betrays him with depleted money rolls stuffed with dollar bills and he attacks her, only for her to escape. Stan then escapes after Ritter blows off part of his ear with a pistol and taunts him. On his way down and traveling the hobo circuit, Stan sees ads touting Madame Zeena. As he descends into alcoholism and despair, a flashback reveals he killed his father by intentionally exposing him to hypothermia before burning his house down.

Stan finds a new carnival, and as he talks to the owner he spies an embalmed freak-in-a-jar on his desk, and the owner explains he purchased it from the old carnival which Stan used to belong to, which had disbanded. Realizing he is not going