As the intense summer heat envelops South Florida, Ned Racine, an inept lawyer with a knack for attracting trouble, finds himself caught in a passionate affair with Matty Walker. Unbeknownst to him, Matty's husband, Edmund, is away on business, granting her the freedom to explore their forbidden romance. One fateful evening, while visiting the luxurious Walker mansion, Ned stumbles upon Mary Ann Simpson, an old high school friend of Matty's, who is deceptively pretending to be the woman he desires.
As the layers of deceit begin to peel back, Matty confesses her wish to divorce Edmund, although a prenuptial agreement threatens to leave her nearly broke. Her drastic solution? A plan to murder Edmund and stage it as an accident, allowing her to claim his substantial inheritance. Eager to seize the chance for wealth, Ned suggests they take matters into their own hands and enlists the help of Teddy Lewis, a dubious explosives expert from his past. Teddy provides Ned with a small incendiary device and warns him of the perilous undertaking.
After executing the plan to eliminate Edmund, Ned and Matty dispose of his body, making it appear as if he tragically lost his life in a fire he attempted to start himself. Following this orchestration, Edmund's attorney contacts Ned about a new will purportedly drafted by the deceased, with Mary Ann as a witness. Trouble arises when the will is found to be incorrectly executed, leaving Matty poised to inherit everything while denying any share to Edmund's sister. Aware that the police will likely scrutinize the sudden shifts in the will, especially given his own problematic background and Mary Ann's entanglement, Ned finds himself in hot water.
As suspicions mount, Ned becomes ensnared within a tangled web of lies and legal complexities. Central to this mystery is the often-confounding rule against perpetuities, which may become pivotal in untangling the convoluted occurrences. Paranoia grips Ned as two of his acquaintances—Peter Lowenstein, an assistant deputy prosecutor, and Oscar Grace, a police detective—start to link him to Edmund's demise.
The investigation digs deeper, revealing a trail of damning evidence, such as Edmund's missing eyeglasses, which only worsens Ned's already precarious alibi. Moreover, hotel logs indicate numerous unanswered calls from Ned's room on the night of the murder, while the enigma of Mary Ann's whereabouts further muddies the waters of his innocence.
With his nerves fraying, Ned finds an old lawyer acquaintance who mentions being recommended by someone else, adding another layer of anxiety as he realizes Matty had likely known of his limited legal skills all along. Teddy's ominous hints, regarding inquiries from the police in connection with an arson investigation, only serve to deepen Ned's anxiety as the truth remains elusive.
Meanwhile, Matty contacts Ned, claiming to possess Edmund's missing eyeglasses in her boathouse. On a shadowy night visit to retrieve them, he encounters a suspicious wire connected to the door, leading to a confrontation with Matty herself. Just as Ned asks her to fetch the glasses, Oscar Grace shows up, magnifying the tension of the moment.
In a shocking twist, Matty makes her way towards the boathouse and vanishes just before it detonates, with the remains found subsequently identified as Matty Walker. Confined and desperate, Ned's thoughts spiral as he theorizes that the real Matty Tyler had been fooled into taking on the identity of her adversary, all orchestrated by an unseen hand to eliminate threats to the plan.
As he languishes behind bars, Ned comes into possession of Matty Walker's high school yearbook, revealing damning evidence that Mary Ann Simpson and Matty Tyler share an uncanny resemblance. With the yearbook’s striking details—including the ominous nickname "The Vamp" and a chilling ambition to be wealthy and live lavishly in an exotic realm—Ned realizes that the true Mary Ann has escaped with her husband’s fortune, leaving behind a life of struggle for one of luxury on a sun-kissed shore.
