A man is put on trial for the alleged murder of his wife.

After finding her mistress Daphne Stroud unconscious, Daphne's devoted maid Mrs. Tucker rushes her to the hospital and reports to the police that Daphne's husband Roderick is fleeing the country. When Tucker finds an unidentified stranger's body in Daphne's hospital room due to a paperwork mix-up, Tucker so hates Roderick that she identifies the corpse as Daphne. Later at the airport, Det. Sgts. Horne and Bolton find Roderick with his mistress, Mrs. Elizabeth Anders, and tell him of Daphne's death. Although Roderick states, "I've killed her" and admits to giving Daphne sleeping tablets, he claims that her death is impossible and disavows his relationship with Elizabeth. While Roderick awaits trial for murder in jail, potential jury members receive their summons while going about their daily life. Oliver Bracket reads about the case and condemns Roderick for his philandering, but his wife reminds him of his own affair and then forgives him for it. At the Prynne home, meek Lucy is so enraged by her demanding mother that she contemplates killing her, while playboy Cyril Gaskin tells his mistress that he can love both her and his wife at the same time. When undertaker Bonaker receives his summons, he laments that the only certainty for man is taxes and death. Meanwhile, Roderick's close friend Mark employs solicitor Antony Harcombe Q.C., who demands that Roderick tell him the truth. Roderick recounts the events that led up to his wife's death: A music critic, Roderick took a professional interest in young opera singer Elizabeth after meeting her in the recording studio. Learning of her husband's interest in the rising star, Daphne tortured her husband with jealous accusations. Fleeing Daphne's possessiveness, Roderick fell in love with Elizabeth and sent Mark to issue divorce papers to his wife. During their meeting, Daphne tells Mark that Roderick fled because of her jealousy, but refuses to grant the divorce. Later, when a drunken and hysterical Daphne claims that she is pregnant and reaches for a gun to kill herself, Roderick struggles for the firearm, which discharges several shots without injury. Tucker rushes to the room, only to be sent away by Roderick. Daphne then admits that she lied about the pregnancy. Back in the jail cell, Roderick explains that the sleeping tablets Daphne used were from Elizabeth, who had given them to him for his insomnia. When Harcombe accuses Roderick of blaming Elizabeth for the murder and involving Mark in the deception, Roderick's adamant denial and subsequent order for Harcombe to leave convinces the solicitor that his client is honest. While Roderick has his own view of the events of the night of the death, the truth reveals another scenario. Daphne's doctor suggested to Roderick that rest was essential to her recovery from the nervous breakdown, but told only Tucker to give Daphne two sleeping tablets in milk that evening. When Tucker delivers the milk to Daphne's room, Roderick takes the glass and then pushes Tucker out of the room before the maid can explain its contents. Roderick then inadvertently adds two pills into the already medicated milk. Although Roderick tells Harcombe that earlier that night Daphne finally agreed to the divorce and encouraged him to leave with Elizabeth, the lawyer reminds Roderick that they have no proof of these intentions. On the first day of the trial, the prosecuting attorney general suggests that Roderick's motive was his guilty passion and calls Bolton to the stand. Bolton testifies that when Roderick was apprehended, he said, "I killed her," but during his cross examination, Harcombe presents the officer's arrest notes, which reveal that Roderick said, "I've killed her." Harcombe suggests that the sentence implies a number of different meanings: "Have I killed her?" or "I have killed her indirectly." In the jury room during a recess, Cyril is confident about Roderick's guilt, while Bonaker insists that they have not heard all sides. When one jury member explains her conscientious objection to hanging, the group discusses the government's impending abolishment of capital punishment, which would mean that Roderick would become the "last man to hang" if found guilty. After the trial resumes, the attorney general accuses Elizabeth of luring a married man away from his wife, but the judge reminds the jury that moral judgment must not color their decision. Elizabeth corroborates Roderick's story and insists that she has no time or interest in marriage and therefore, Harcombe argues, no motive for murder. Next on the witness stand, Tucker claims that, despite her being pushed from the room, Roderick heard her speak of the pills. When she says "he murdered her," Harcombe insinuates that the maid's love was so strong for Daphne that she would lie to punish anyone who had hurt her. Tucker then suggests that Roderick, on the day of the shooting, dropped the gun in his pocket to hide any evidence of his attempt on Daphne's life and acknowledges that she wants to see him hang. The next day Roderick admits to the jury that he thought that he had driven his wife to take her own life, but insists that he did not know about the doctor's tablets. Upon further questioning, Roderick professes his love for Daphne, prompting Elizabeth to leave the courtroom. When the judge addresses the jury for the final time, he reminds them to consider the current debate in the House of Commons about capital punishment. After hours of deliberations in the jury room, Bonaker is the only jury member who believes Daphne's death was accidental. After Cyril insists that Roderick could not have heard Tucker, Bonaker reenacts the events using the jury room door, prompting the jury to realize that Roderick could not have heard the maid. Bonaker asks the jury if they have ever been maddened to the point of murder and reminds them that the existence of the feeling does not make one a murderer. After Bracket remembers his wife's forgiveness for his affair and Cyril admits that he could have considered murdering his wife if he loved another woman, Bonaker asks for another vote. When the court resumes, the jury delivers a "not guilty" verdict. Upon his return home, Roderick finds a remorseful Tucker, who admits to praying for his death and explains that Daphne is alive. After she takes Roderick to his wife and wishes him luck, Tucker is taken into police custody to await her fate.