A British lawyer gets caught up in a couple's tangled marital affairs when he defends the husband for murder.

Following a lengthy hospital stay for a near-fatal heart attack, famed London barrister Sir Wilfrid Robarts returns to his combined office and lodgings near The Old Bailey, accompanied by his overbearing nurse, Miss Plimsoll. Sir Wilfrid chafes at her constant vigilance and becomes despondent at the thought that he may no longer be able to try criminal cases. That afternoon, Mayhew, a friend and solicitor, arrives unannounced to discuss an urgent case. Despite a verbal scolding from Miss Plimsoll, Sir Wilfrid speaks with Mayhew and his client, Leonard Vole. Mayhew fears that Leonard will soon be charged with the stabbing murder of Mrs. Emily Jane French, a wealthy widow who was a friend of Leonard, and whom he is known to have visited the day she was killed. Upon questioning, the personable Leonard relates that he was in the army during World War II and stationed in Germany, where he met Christine, a German actress whom he married and brought home to England. Admitting that he has been unemployed for months, Leonard says that he is an inventor who has been trying to get financing for his revolutionary new eggbeater. He then describes two accidental meetings with Mrs. French, after which they became friends. Charmed by Leonard's straightforward manner and sheepish confession to having hoped that Mrs. French would finance his invention, Sir Wilfrid nonetheless turns the case down on doctor's orders. He then suggests fellow barrister Brogan-Moore, whom he has his faithful assistant Carter summon. Sir Wilfrid pressures Leonard on details of the night of the murder and his relationship with Mrs. French. Though increasingly emotional, Leonard does not change his story, impressing Sir Wilfrid with his innocence. When Brogan-Moore arrives, Sir Wilfrid tells him that the case should be easy, as there was absolutely no motive for Leonard to kill Mrs. French, who might have given him money if she had lived. Brogan-Moore then reveals that in Mrs. French's will, which has just been opened, she left Leonard £80,000. Leonard reacts happily to news of the legacy until suddenly realizing its implication. Moments later, the police arrest him. After Leonard is taken away, Brogan-Moore, who is not convinced of his innocence, relates that Christine is his only alibi. As Sir Wilfrid is about to go take a rest, Christine appears at the office, surprising him with her sophistication and cool detachment. Although she confirms Leonard's alibi, she implies that he asked her to lie and has not been truthful about his relationship with Mrs. French. Sir Wilfrid is shocked when she matter-of-factly states that Leonard "has a way with women," then announces that she and Leonard are not legally married because she never divorced her German husband. After she promises to be very convincing on the witness stand, even if lying, Brogan-Moore concludes that the case is hopeless. Sir Wilfrid, however, believing in Leonard's innocence, takes the case. Just before the trial, Sir Wilfrid visits Leonard in jail and reads a statement from Mrs. French's housekeeper, Janet McKenzie, in which she swore that Leonard had helped Mrs. French draft a new will. In answer to a question the police had about a cut on his finger, Leonard says that he got the cut while slicing a loaf of bread, something Christine can confirm. Leonard asks why Christine has not come to visit him, then breaks down, saying that he cannot get through the trial without her. On the day of the trial, Sir Wilfrid's fragile health causes him to miss the opening moments, but he soon arrives with a flask of brandy camouflaged for Miss Plimsoll's benefit as cocoa. Sir Wilfrid objects strenuously to every point made by Crown Prosecutor Mr. Myers, while Miss Plimsoll observes from the spectators' gallery, discussing the case with a young woman. Following damning testimony by the first few witnesses, Janet remains steadfast about her previous statements about the night of the murder and the day that she overheard Leonard and Mrs. French discussing the will. However, Sir Wilfrid successfully establishes that Janet had been Mrs. French's beneficiary in the previous will and has a hearing problem that would make it difficult for her to discern voices behind a closed door. On the third day of the trial, Christine is called to testify. Upon learning that their marriage was never valid and hearing Christine testify that he came home on the night of the murder and said "I've killed her," Leonard breaks down in anguish as women in the courtroom express their disdain for Christine. During an emotional cross-examination, Sir Wilfrid establishes the pattern of lies Christine has told, accusing her of being a habitual liar, but she will not be shaken from her testimony. When the crown rests its case, Sir Wilfrid calls his only witness, Leonard, who steadfastly affirms that he is not guilty. Under cross-examination, Myers brings up new evidence that Leonard and an unidentified young woman had visited a travel agent on the day of Mrs. French's murder and were interested in deluxe cruises. Leonard says that he hardly knew the girl and was merely asking for brochures for fun, then becomes hysterical over the horrible nightmare in which he has found himself. That evening, Sir Wilfrid ponders Christine's testimony, telling Mayhew that he cannot understand why she lied. Just then, he receives a phone call from an anonymous Cockney woman who says she has "the goods" on Christine and demands that Sir Wilfrid meet her at Euston Station. Sir Wilfrid immediately goes to meet the woman, who snarls her hatred of Christine, and after Sir Wilfrid gives her £40, hands over a packet of "juicy" letters from Christine to a man named Max, who she says had been her lover before falling in love with Christine. She refuses to give her name, or Max's last name, then disappears after showing Sir Wilfrid a scar on her face, which she said came from Max. The next day in court, as Myers begins his closing statement, Sir Wilfrid interrupts to recall Christine. Over Myers' objections, the judge allows Christine to retake the stand. Now Sir Wilfrid confronts her with the content of the letters which stated, in her own hand, that she was planning to place the blame for Mrs. French's murder on Leonard so that she could be free to be with Max. Christine screams out "Lies, all lies," but Sir Wilfrid tricks her into confirming that the letters were hers. The jury quickly returns a not guilty verdict, but Sir Wilfrid begins to think that everything was "too neat." While Leonard is retrieving his things from the bailiff, Christine comes back into the near empty courtroom, seeking refuge from the crowd of angry spectators. When Sir Wilfrid warns that she will go to jail for perjury, she demurs, saying that the testimony she gave was the truth, not because she knew that Leonard was innocent, but because she knew he was guilty. She then reveals that she did what she had to because she loves Leonard and the jury never would have believed supportive testimony from a loving wife. She then assumes the Cockney woman's accent and reveals that Max and the letters were figments of her imagination. Now Leonard re-enters the courtroom and blithely says that he knew Christine was planning something but not what. As he is promising to pay for Christine's defense, Miss Plimsoll and Diana, the young woman from the spectator's gallery, enter the courtroom. When Diana throws herself into Leonard's arms and announces that she is his girl, Christine is stunned. Leonard then coolly tells Christine that her saving his life pays him back for taking her out of Germany. Christine then grabs the murder knife still lying on the table and plunges it into Leonard. After Miss Plimsoll examines the body and announces "she killed him," Sir Wilfrid responds, "she executed him." As Sir Wilfrid ponders the case, Miss Plimsoll tells Carter to cancel his planned Bermudan vacation. After she hands Sir Wilfrid his court wig and reminding him not to forget his flask of brandy, he puts his arm around her as they leave the courtroom together.