The central twist of "Anatomy of a Murder" is not a last-minute revelation but the gradual and unsettling realization that the entire defense may be a carefully constructed lie. Defense attorney Paul Biegler subtly coaches his client, Lt. Manion, on the concept of "irresistible impulse," a form of temporary insanity defense. Manion, who initially claims to have no memory of the murder, conveniently adopts this as his official story. The brilliance of the film lies in its ambiguity; it never explicitly confirms whether Manion was genuinely insane or if he was coached into a phony story.
During the trial, the prosecution, led by Claude Dancer, attempts to paint Laura Manion as a promiscuous woman who may have consented to a sexual encounter with the victim, Barney Quill. A key piece of evidence, Laura's missing panties, becomes a central point of contention. The case takes a surprising turn when it is revealed that the innkeeper Mary Pilant, whom Dancer had accused of being Quill's mistress, is actually his illegitimate daughter. This revelation discredits the prosecution's theory about her motives.
Ultimately, the jury finds Lt. Manion not guilty by reason of insanity. The final, cynical twist comes after the trial. Biegler and his partner Parnell go to collect their fee from the Manions, only to find they have skipped town. Manion has left a note for Biegler, stating that he had to leave due to an "irresistible impulse," mockingly throwing the very legal defense that acquitted him back in his lawyer's face. This ending strongly implies that Manion was guilty and manipulated the legal system to get away with murder, leaving Biegler and the audience to grapple with the unsettling nature of the verdict. It becomes clear that Laura was likely aware of the deception, and her earlier fear of her husband's temper suggests a more complex and potentially abusive relationship.