A small-town lawyer gets the case of a lifetime when a military man avenges an attack on his wife.

Ever since losing his bid for reelection as the district attorney of Iron City, Michigan, attorney Paul "Polly" Biegler has sought solace in his two favorite hobbies, playing jazz on the piano and fishing. One day, when a woman named Laura Manion phones Paul and begs him to represent her husband Frederick, who has been arrested for murder, Parnell McCarthy, a lapsed lawyer who views life through the bottom of a liquor bottle, urges his friend Paul to accept. Because Paul had been on a fishing trip at the time of the murder, Parnell tells him about the case. Manion, an army lieutenant serving at a nearby base, has been charged with killing Barney Quill, a bartender who allegedly raped Laura. The next morning at the jailhouse, Paul meets Laura, who is sporting a black eye that she claims was inflicted by Quill during the rape. When Manion insolently asserts that the murder was justified by the rape, Paul experiences doubts about taking the case. After completing his conference with the Manions, Paul relates his feelings to Parnell, who urges him to take the case because he needs the money. Following Parnell's advice that it is a lawyer's duty to "guide" his client to the correct defense, Paul coaches Manion to say that he was insane at the time of the murder. Upon returning to his office, Paul finds the seductively dressed Laura waiting to see him. When he asks her to recount the night of the rape, she states that Quill offered her a ride home, but when they found the gates to her trailer park locked, he pulled off the road and raped and beat her. After Laura leaves, Paul asks Parnell to work with him on the case, but warns that he must "lay off the booze." Paul then informs Manion that he will represent him, and Manion, pleading poverty, asks the attorney to accept a promissory note. Later, as Laura flirts with Paul outside the jailhouse, Paul warns that her husband is watching. Knowing that Manion is insanely jealous, Laura flinches. Paul then proceeds to the bar in Thunder Bay where Quill worked. Although bartender Alphonse Paquette curtly responds to his questions, Paul ascertains that Mary Pilant, who manages Quill's Thunder Bay Hotel, now runs the bar. Paul's secretary, Maida Rutledge, and Parnell then try to unearth information about Mary and learn that she recently moved to Thunder Bay from Canada and that Quill was fiercely protective of her. That evening at a nightclub, Paul sees a rowdy Laura dancing with some soldiers and takes her home, warning that she must appear demure in order to give credibility to her husband's case. Laura shocks Paul when she responds that she would be glad if her husband was convicted, because then she could get away from him. With two days left before the start of the trial, Manion confers with army psychiatrist Matthew Smith, who diagnoses that he is suffering from "disassociative reaction" or, in layman's terms, an irresistible impulse to shoot Quill. While scouring the law books for a precedent on which to base their case, Parnell and Paul come upon an 1886 ruling in which the court concluded that the defendant was forced to commit a crime because of an impulse he was powerless to control. The following Monday, as the court is convened by visiting judge Weaver, Laura enters the room, dressed in a baggy suit, horn rimmed glasses and a hat. Mitch Lodwick, the relatively inexperienced new district attorney, has requested that Claude Dancer, an assistant attorney general from the "big city," serve as co-counsel. As testimony begins, Paul charges that the prosecution is suppressing evidence about the rape in order to insure that his client is convicted. When George Lemon, the manager of the trailer park, testifies that Manion admitted murdering Quill, Paul steers him into acknowledging that Laura had been severely beaten and that screams had been heard at the park gates on the night of the murder. Sgt. James Durgo then takes the stand, and Paul tricks him into admitting that the district attorney instructed him to substitute the phrase "some trouble" for rape. Overruling Mitch's objections about introducing the rape, the judge allows Paul to continue his line of questioning. Durgo then avers Laura was raped, although her panties were never found at the scene of the crime. Incensed by the turn of events, Dancer takes over the questioning and tries to portray Laura as a seductress. Paul rebuts his charge by stating that Laura's beauty drove her husband to murder the man who defaced her. Parnell had left the courtroom during the trial, and once court is adjourned for the day, Durgo informs Paul that his friend has been injured in an automobile accident, then takes him to Parnell's hospital room. There Parnell, who does not possess a driver's license, tells Paul that he drove to Canada to investigate Mary's past and has learned that she was Quill's illegitimate daughter. When the trial reconvenes, Paul tries to establish Laura's veracity by introducing the fact that she swore to her husband on a rosary that she was raped. As Dancer calls Laura to the stand, Mary enters the courtroom. After eliciting that Laura had previously been divorced, Dancer establishes that she has been ex-communicated from the Catholic Church and therefore her oath was meaningless. Dancer then suggests that her panties were never found because she was not wearing them and implies that she lied about being raped to prevent her insanely jealous husband from beating her. Paul then calls Dr. Smith to the stand to testify that the shooting was a case of "disassociative reaction." In his cross-examination of Smith, Dancer asks if Manion would have known right from wrong in that state. When Smith answers in the affirmative, Dancer smugly calls for a conference in the judge's chambers and argues that because Manion knew right from wrong, he cannot be adjudged legally insane. Paul then hands the judge a law book and asks him to turn to the 1886 court ruling that established the precedent for Manion's defense. Outfoxed by a man he incorrectly deduced was a folksy attorney, Dancer returns to the courtroom to call a surprise witness, Duane "Duke" Miller, an inmate who had been incarcerated with Manion. When Duke swears that Manion boasted that he fooled his attorney, his psychiatrist and that he will also fool the jury, Paul asks for a copy of the inmate's record. Paul then impugns his testimony by reading the litany of crimes for which he has been convicted, calling him an inveterate criminal and liar. After Paul returns to his seat, Maida whispers that Mary, who earlier had left the courtroom, is waiting in the hall to talk to him. Once Paul ushers Mary into the courtroom, she takes the stand and testifies that after learning of the missing panties, she returned to the hotel where she found them in the laundry chute next to Quill's room. When Dancer, believing that Mary was Quill's mistress, argues that someone else could have put them in the chute and that Mary brought them to court in revenge for Quill's interest in Laura, Mary invalidates his argument by revealing that she was Quill's daughter. The jurors adjourn after closing arguments and soon return with a not guilty verdict. After Manion is released, Paul and Parnell drive to the trailer park to collect Manion's promissory note. Upon arriving at the park, they are greeted by Lemon, who informs them that the Manions have gone. Lemon hands Paul a note from Manion, and tells him that Laura was in tears as the trailer pulled out. In the note, Manion explains that he was seized by an "irresistible impulse" to leave.