Charlie Johnson, semi-retired psychiatrist, homeward-bound by train from L.A., encounters Elaine Winters, his lover prior to his elopement 27 years earlier. Now Elaine Fogarty, wealthy and widowed, she lives a few miles from Charlie and his wife Jan in Santa Carlita, California. Elaine is hot to renew the relationship; Charlie is tempted, because Jan, still beautiful at 52, is bored with the marriage, and the two have little to do with each other, sexually or intellectually. But they're just getting by on his limited income, and divorce would be a financial disaster.

Over coffee, Elaine is flattering and makes clear her interest in more than talk. Meanwhile at her club—Charlie's not a member—Jan meets Raul Garcia, charming and 10 years her junior who says he's an Ecuadorian import-export consultant, and accepts his invitation to brunch. Charlie, against his better judgment, meets Elaine at her half million dollar apartment; after lunch she confirms his belief that women over 40 are sexiest. But some of her comments are stran≥ she claims Charlie's destined to be hers and urges him to get rid of Jan. He admits to dissatisfaction with his work, and Elaine assure him he'd never have to work if he threw in his lot with her.

On county psychiatric wards, Charlie interviews psychotic patients, and it’s evident society has little defense against certain types of delusional—and dangerous—people. "Fight fire with fire," Charlie advises a nurse.

Captivated by Raul, Jan accepts his dinner invitation, and three weeks later, she and Raul spend the weekend together and become lovers, bringing joy and excitement to her life.

Meanwhile Charlie's a regular visitor to Elaine's bed. But he's uneasy when she says God communicated to her that it's His will she have Charlie and displays her gun collection and boasts of her marksmanship. He tells Elaine that, much as Jan irks him, he can't bring himself to break with her, but if Jan left him, there'd be no problem.

Elaine isn't the chance encounter Charlie thinks she is. She'd hired a detective agency to track him down, then arranged the encounter on the train. Now, to obtain more information on Jan and her lover, Raul, Elaine hires a PI, bi-cultural Jorge Hawkins. He gets the information but also uncovers reason to believe Elaine murdered her husband.

And Raul's not a wealthy Ecuadorian. He's Ramon Gomez, former sales rep for a Puerto Rican pharmaceutical firm who, when offered a chance to make half a million by Orlando, a Colombian drug dealer, had persuaded four friends to each kick in $10,000. When Ramon caught his wife in bed with his best friend, he killed them both and absconded with the money. Now, funds low and thinking Jan, a doctor's wife, must have money, he arranged their meeting in hopes of swindling her. When he discovers the truth and Elaine offers him money to seduce Jan away from Charlie, Raul readily agrees.

Charlie, "addicted" to Elaine, spends his free time with her, but chooses to ignore her increasingly bizarre statements. She describes her deal with Raul, but says, if he's unsuccessful, she as God's tool must kill Jan to carry out God's command that she have Charlie. Charlie finally admits to himself that Elaine's delusional and dangerous but merely urges her to rely on Raul's charm rather than her guns. She's insistent he break with Jan but grudgingly agrees to wait. Charlie tries to persuade Jan she should get away but can't bring himself to tell her the real reason: she'd be devastated if she learns Raul's true motive. Convinced Charlie wants to pose as a deserted husband and seize their property, Jan refuses to leave.

In a restaurant, Raul and Jan are confronted by his former friends—now sworn enemies —who are, at the behest of Orlando, en route to arrange a drug deal in Mexico. Tumult ensues; Raul, pursued by revenge-minded Luis, Miguel, and Carlos, fle