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Assistant district attorney Cleve Marshall goes on a drunken binge and misses celebrating his anniversary with his wife Pamela.  Left alone in chief investigator Miles Scott's office, Cleve drunkenly pursues Thelma Jordon, an alluring and confident woman, who is reporting an attempted burglary at her elderly aunt Vera's house.  Thelma agrees to join Cleve for a drink after he offers to fix a parking ticket for her.  He stays with her until late that night, when she throws him out of her car for proclaiming his love.

The next day, Cleve proffers an apology and meets Thelma for dinner, as Pamela and their children have taken up their usual summer residency at the beach.  Cleve is drawn to Thelma, who confesses she is lonely, and agrees to see her again.  Unknown to Cleve, Thelma is also seeing a shadowy character named Tony Laredo, but once she has started dating Cleve regularly, she lies that she is in a loveless marriage with Tony.

One night, Vera is shot and killed while investigating a noise in her house, and Thelma is delayed in picking up Cleve for a weekend holiday.  Cleve, who uses a pseudonym, calls the house shortly after Vera is shot, and Vera's butler listens on the extension as Thelma urges Cleve to come immediately.  After Cleve arrives, Thelma admits that she tried to cover up the murder, because she once wrote to Tony about Vera's expensive emerald necklace and fears he may have killed her.  Cleve insists that Thelma restore her fingerprints to the room, and ducks out just as the butler enters.  Cleve retreats to the beach house with Pamela, who forces him to admit that he is having an affair.

Cleve is called to help investigate Vera's murder, and suspicion soon falls on Thelma, who begs Cleve to prosecute the case in order to protect her.  Apparently unaware of Thelma and Cleve's indiscretion, Miles is unable to identify Thelma's mysterious caller, who has been dubbed "Mr.  X," and who was seen by the butler leaving the crime scene.  Based on Thelma's fingerprints, the recovered jewels, and a newly rewritten will which favors Thelma, a grand jury indicts her for Vera's murder.  Cleve anonymously hires lawyer Kingsley Willis to defend Thelma, and cleverly arranges for the disqualification of the district attorney, so that he will be made prosecuting attorney.

During the trial, Cleve purposely antagonizes the jury in order to lose the case.  Although the trial exposes Thelma's sordid past--the gambling, blackmail and illicit relationship with Tony--it fails to sway Cleve's love.  Willis makes a convincing case for Thelma's innocence, and "Mr.  X" is never identified as Cleve.  After the trial, Thelma immediately packs to leave with Tony, who planned the robbery and now intends to live off Thelma's inheritance.  Cleve pays Thelma a final visit, during which Tony forces her to admit that they set Cleve up, and that she murdered Vera.  Tony knocks Cleve out and leaves with Thelma, but when Thelma attacks Tony with a hot cigarette lighter, their car crashes through a barrier over a cliff.  As Thelma lies dying in the hospital, she admits the full truth to Miles, but still refuses to identify "Mr.  X."  Thelma dies, and Miles realizes that Cleve is "Mr.  X."  Having already given his resignation to the district attorney, Cleve bids Miles farewell, asking him to tell Pamela that he will see her later.