During a particularly intense Florida heatwave, inept central Florida local lawyer Ned Racine meets and begins an affair with Matty Walker. She is married to wealthy businessman, Edmund Walker, who is home only on the weekends. One night, Ned arrives at the Walker mansion and playfully propositions a woman he mistakes as Matty. The woman, who bears a strong resemblance to Matty, is Mary Ann Simpson, Matty's visiting high school friend. Soon after, Matty tells Ned she wants to divorce Edmund, but their prenuptial agreement would leave her with little money. She also says that she wishes her husband dead. Eventually, Ned suggests murdering Edmund so Matty can inherit his wealth. To that end, he consults a shady former client, Teddy Lewis, an expert on incendiary devices, who supplies Ned with a bomb while strongly discouraging him from whatever he is scheming.
Ned, aided by Matty, kills Edmund and moves the body to an abandoned building connected to Edmund's business interests. Ned detonates the bomb to make it look like Edmund died during a botched arson attempt. Soon after, Edmund's lawyer contacts Ned about a new will that Ned supposedly drafted for Edmund and which was witnessed by Mary Ann Simpson. The new will was improperly prepared, making it null and void, with the result that Matty inherits Edmund's entire fortune, disinheriting Edmund's surviving sister. Ned immediately realizes that Matty forged the will, capitalizing on his past malpractice troubles, and knowing that it would be nullified, leaving her the sole beneficiary. Ned also knows the police will view the sudden change to Edmund's will shortly before his death as suspicious.
Following the will reading, two of Ned's friends, assistant deputy prosecutor Peter Lowenstein, and police detective Oscar Grace, seem to suspect that Ned may be involved in Edmund's death. Edmund's eyeglasses, which he always wore, are missing. Lowenstein also informs Ned that on the night of the murder, hotel phone records show that repeated calls to Ned's room went unanswered, thereby weakening his alibi, and suggests that Mary Ann Simpson cannot be found, despite police efforts.
Already nervous over the mounting evidence implicating him, and questioning Matty's loyalty, Ned happens upon a lawyer who once sued him over a mishandled legal case. The lawyer says that to make amends, he recommended Ned to Matty Walker, and admits he told her about Ned's limited legal skills. Later, in a jailhouse visit, Teddy tells Ned about a woman wanting an incendiary device, and that he showed her how to booby trap a door. Teddy also tells Ned that the police have been asking questions about the apparent arson.
That night, Matty calls Ned and says that Edmund's glasses are in the Walker estate boathouse. Ned arrives later that night and spots a long twisted wire attached to the door. When Matty arrives, after a confrontation, Ned asks her to retrieve the glasses. In the meantime, Det. Oscar Grace has arrived and is observing their interaction. To prove herself, Matty walks toward the boathouse and disappears from view; the boathouse explodes. Afterward, a body found inside is identified through dental records as Matty Walker (née Tyler).
Now in prison, Ned, having realized Matty duped him, tries to convince Oscar Grace that she is still alive. He believes the woman he knew as "Matty" assumed the real Matty Tyler's identity in order to marry and murder Edmund for his money. Ned surmises that the "Mary Ann Simpson" that Ned met had discovered the scheme and was blackmailing Matty, only then to be murdered. Had Ned been killed in the boathouse explosion, he reasons, the police would have found both suspects' bodies.
In the epilogue, Ned, in prison, obtains a copy of Matty's high school yearbook: in it are photos of Mary Ann Simpson and Matty Tyler, confirming his suspicion that Mary Ann assumed Matty Walker's identity. Below Mary Ann's photo is the nickname "The Vamp" and "Ambition—To be rich and live in an exotic land".
In the final scene, the real Mary Ann (Matty) is seen lounging on a tropical beach alongside a Portuguese-speaking man.
