During a particularly intense Florida heatwave, inept 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. He consults a shady former client, Teddy Lewis, an expert on incendiary devices, who supplies Ned with a bomb while strongly encouraging him to abandon 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 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, resulting in Matty inheriting Edmund’s entire fortune while disinheriting his surviving blood relatives. Matty later reveals to Ned that she forged the will, knowing that it would be nullified and that she would be the sole beneficiary. Ned knows the police will view the sudden change to Edmund’s will shortly before his death as suspicious.
Two of Ned’s friends, assistant deputy prosecutor Peter Lowenstein, and police detective Oscar Grace, suspect that Ned may be involved in Edmund’s death. Edmund’s eyeglasses, which he always wore, are missing, and Mary Ann Simpson has also disappeared. 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.
Lowenstein 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. Teddy tells Ned about a woman wanting an incendiary device, and that he showed her how to booby trap a door. 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, Ned asks her to retrieve the glasses. Matty walks toward the boathouse and disappears from view; the boathouse explodes. 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. The “Mary Ann Simpson” that Ned met had discovered the scheme and was blackmailing Matty, only to also be murdered. Had Ned been killed in the boathouse explosion, the police would have found both suspects’ bodies. Sitting in his cell, Ned 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.