On January 9, 1991, in Dallas, Texas, Nancy Lyon, Glynnis O'Connor is rushed to the hospital where she ultimately dies from arsenic poisoning. Detective Ortega, Shawn Elliott, and A.D.A. Sims begin to investigate the case. The pair question Nancy's wealthy family, the Dillards, who explain that Nancy and her husband, Richard Lyon, Richard Thomas had recently reconciled after his earlier departure to be with his mistress. Nancy had suspected that Richard was slowly poisoning her but kept quiet, too embarrassed to involve the police.
Nancy's brother Bill Dillard Jr., Gary Frank, is convinced that Richard killed Nancy to inherit her fortune. The Lyons' nanny, Ellen, is described by family members as a devoted caregiver who sees Richard as a loving father to the couple's two children. Flashbacks reveal that both Nancy and Richard were architects, with Nancy's far more successful career frequently keeping her away from home and placing strain on their marriage. The couple also argued over Nancy's family, who often looked down on Richard.
The police learn that Richard Richard Thomas had recently purchased arsenic, which he says was for a fire-ant infestation in the house. Richard suggests his lawyer, Guthrie, Matthew Posey look into the possibility that Nancy may have committed suicide. Richard recalls how Nancy was full of life when they first met in college and insists they were happily married until a particular family trip to Arizona, during which Nancy visited Bill Dillard Jr., Gary Frank who was in a treatment center for drug and alcohol problems. After that trip, Nancy began isolating herself, quit her job and started seeing a therapist. Richard says it was at this time that he began an affair and filed for divorce, although he later returned to Nancy for the sake of their children.
Meanwhile, the police receive anonymous letters claiming that Bill is responsible for poisoning Nancy. As they are unable to identify the sender, Richard remains the police's prime suspect. After a grand jury indicts Richard, he is arrested and put on trial for Nancy's murder. During the trial, the defense attorney Guthrie is criticized for overlooking Bill as a potential suspect. It comes to light from Nancy's therapy sessions that Bill and Nancy had an incestuous relationship when they were children. Guthrie also presents handwritten notes found among Nancy's belongings in which she writes about how grateful she is to Richard for saving her from Bill, and her fears that Bill might have sexual desires toward her daughters. Despite Richard's insistence that he is innocent, the jury finds him guilty of first-degree murder and he is sentenced to life imprisonment by the Trial Judge, Ralph Wilcox.
