Kevin Lomax, a defense attorney from Gainesville, Florida, has never lost a case. As he defends a schoolteacher, Lloyd Gettys, against a charge of child molestation, Kevin belatedly realizes his client is guilty and local reporter Larry warns him a guilty verdict is inevitable. However, through a harsh cross-examination, Kevin destroys the victim’s credibility, securing a “not guilty” verdict.
Subsequently, a representative of a New York City law firm offers Kevin a large sum of money to assist a jury selection. After the jury delivers a not guilty verdict, the head of the firm, John Milton, offers Kevin a large salary and an upscale apartment if he joins the firm. Kevin accepts the job, and he and his wife Mary Ann move to Manhattan. He is soon spending most of his time at work, leaving Mary Ann feeling isolated. Kevin’s fundamentalist mother, Alice, visits New York City and suggests they both return home, but Kevin refuses.
When billionaire Alex Cullen is accused of murdering his wife, his stepson, and a maid, Milton assigns the high-profile case to Kevin. This demands more of Kevin’s time, further separating him from Mary Ann, and he begins to fantasize about his co-worker, Christabella. Mary Ann begins seeing visions of the partners’ wives becoming demonic, and has a nightmare about a baby playing with her removed ovaries. After a doctor declares her infertile, she begs Kevin to return to Gainesville, of which he disapproves. Milton suggests Kevin step down from the trial to tend to his wife, but Kevin claims that if he steps down and she recovers, he may resent her for costing him the case.
Eddie Barzoon, the firm’s managing partner, is convinced that Kevin is competing for his job when he discovers Kevin’s name is on the firm’s charter. Surprised, Kevin denies any knowledge of this and Eddie threatens to inform the United States Attorney’s office of the law firm’s activities. Kevin tells Milton about Eddie’s threats, but Milton dismisses them; Eddie is then beaten to death by vagrants with demonic appearances. Mary Ann witnesses this, disturbing her further.
While preparing Melissa Black, Cullen’s secretary, to testify about Cullen’s alibi, Kevin realizes she is lying and tells Milton he believes Cullen is guilty. Despite this, Kevin proceeds with her testimony and the trial. Afterwards, Kevin finds Mary Ann in a nearby church covered with a blanket. She claims Milton raped and brutalized her, but Kevin believes this cannot be true as he was with Milton in court. Mary Ann drops her blanket, revealing her naked body covered with cuts; Kevin assumes Mary Ann injured herself and commits her to a mental institution.
Alice, along with Kevin and Pam Garrety, Kevin’s case manager from the firm, visit Mary Ann at the institution. After seeing Pam as a demon, Mary Ann hits her with a hand mirror and barricades the room. As Kevin breaks down the door, Mary Ann commits suicide by cutting her throat with a shard of glass. Alice reveals that Milton is Kevin’s father; Kevin leaves the hospital to confront Milton, who admits to raping Mary Ann. Kevin fires a pistol into Milton’s chest, but the bullets are ineffective. Milton reveals himself as Satan and Kevin blames him for everything that happened; Milton explains that he merely “set the stage” and that Kevin could have left at any time. Kevin realizes he always wanted to win, no matter the cost. Milton tells Kevin that he wants Kevin and Christabella, Kevin’s half-sister, to conceive a child, the Antichrist. Kevin appears to acquiesce at first, but then abruptly cites free will and shoots himself in the head.
Kevin finds himself back at the recess of the Gettys trial. Choosing to do the right thing, Kevin announces that he cannot represent his client despite the threat of disbarment. Larry pleads for an interview, promising to make Kevin a celebrity. Encouraged by Mary Ann, Kevin agrees; after they leave, Larry transforms into Milton, relishing the sin of vanity.