In the Upper Peninsula of Michigan, we meet small-town lawyer Paul Biegler (James Stewart), a former district attorney whose career took a downturn after losing his re-election bid. Now, he spends his days indulging in fishing, playing the piano, and spending quality time with his alcoholic friend Parnell McCarthy (Arthur O'Connell) and his sardonic secretary Maida Rutledge (Eve Arden).
One day, Biegler is approached by Laura Manion (Lee Remick), the desperate wife of US Army Lieutenant Frederick "Manny" Manion (Ben Gazzara), who has been charged with the first-degree murder of innkeeper Bernard "Barney" Quill. Although Manion does not deny the murder, he insists that Quill had assaulted Laura. Navigating such a complex case appears daunting, particularly since Manion's memory of the incident is hazy, which leads Biegler to consider a defense based on irresistible impulse—essentially a form of temporary insanity.
Biegler’s folksy charm and relaxed attitude disguise a clever legal mind and a flair for courtroom drama that keeps the judge busy maintaining order. However, the defense encounters numerous challenges, especially given the formidable prosecution team led by Claude Dancer (George C. Scott) from the Attorney General's office. Dancer presents high-pressure tactics, creating obstacles in Biegler’s attempts to build a convincing defense.
As the investigation unfolds, details emerge about Barney Quill's past as an ex-boxer and his time spent in federal prison. Quill's bar is managed by Mary Pilant (Kathryn Grant), who stands to inherit his wealth. Biegler spends considerable time with Laura to clarify her account, and her flirtatious demeanor complicates their dynamic, especially when her husband begins to suspect an affair.
The investigation further reveals that Pilant may have been Quill's mistress. It becomes clear that Quill had been drinking heavily and took his frustrations out on Laura, which could justify Manny’s actions. One evening, Biegler feels compelled to confront Laura about her behavior at a jazz club, emphasizing the need for her to remain a model wife until the trial concludes.
During the trial, the prosecution strives to silence any mention of Manion's motive for the murder. Laura's photos from that night are ruled inadmissible, and questions about Quill's actions are met with resistance. Nevertheless, Biegler manages to introduce Laura's account of the assault into the proceedings, leading to heated cross-examinations where Dancer insinuates Laura's flirtatious nature undermines her credibility.
Psychiatrists present conflicting views on Manion's mental state during the murder, while Dancer raises doubts about whether Quill's actions could be categorized as assault. The defense continues to crumble under pressure as the prosecutor details a pattern of jealousy exhibited by Manny.
Amidst all the chaos, it is revealed that Mary Pilant is actually Quill's daughter, a secret she desperately wants to keep. This shocking revelation comes when Pilant is called to testify about Laura’s allegations against her father. The trial climaxes as Manion is ultimately found “not guilty by reason of insanity.”
Afterwards, Biegler resolves to establish a new practice alongside a newly sober McCarthy. When they visit the Manion’s trailer for a signature on a crucial loan, they discover the couple has fled, leaving behind a note from Laura that references Biegler's courtroom argument about irresistible impulse. As Biegler reflects on the case, he mentions that Mary Pilant has hired him to handle Quill's estate, with McCarthy noting that working for her would be a form of "poetic justice."
