Chorus girl Peggy Harper (Betty Hutton) is desperate to marry her fiancé, veteran and lawyer Oliver Clarke (Sonny Tufts), but he does not want to marry until his law practice is successful. Peggy brings Oliver his first client, a hoodlum who has been arrested for stealing hams from a truck, but the principled Oliver refuses to represent Tony because he is guilty. Peggy then calls Wallace Brent (Howard Freeman), the producer of her nightclub show, and agrees to be his personal secretary after he promises to send Oliver business.

When piano movers arrive to repossess her piano because she is late on her payments, Peggy, a compulsive liar, concocts a story that Oliver is insane and terrifies the movers into leaving without the instrument. The next day, Peggy goes to Brent's home office to report for work, and finds Peter (Michael Chekhov), an Eastern European actor, waiting there because he wants to be cast as "Hamlet" in Brent's revival of the William Shakespeare play. Brent mercilessly throws Peter out of his office, and begins pawing Peggy, who tries to distract him by singing, and making a recording of her voice on his phonograph. When the distraction fails to stop Brent, he chases her around the office until she knocks him out with a punch and runs from his house.

Not long after, Peggy returns to Brent's with her mother to collect her coat, and is arrested for stealing $12,000 and killing Brent. Making no reference to the morning's events, Peggy confesses to the murder in the belief that Oliver will become famous by defending her in court. Peggy invents a story which leads police detectives to believe that she killed Brent for the money, but when police actually find the murder weapon in her purse, she gets scared. Although Peggy repudiates her confession, she is not released from jail, and swears to Oliver that she has no knowledge about the knife. The case makes headlines, and Peter visits her in jail, saying that he knows she is not guilty. Peter asks her for some newspaper clippings, but Peggy claims never to have seen them.

At the trial, all the evidence points to Peggy as Brent's killer, and because she is such a compulsive liar, Oliver starts to doubt her innocence. Peggy tells the truth on the stand, then sings for the courtroom, believing it will earn her sympathy. When Peter brings in the recording of Peggy singing for Brent, Oliver realizes that Peggy lied when she told him Brent was harmless. After the jury deliberates for over a day, Peggy is declared not guilty. Although the news is good, Oliver breaks their engagement because he no longer trusts her. As time passes, Oliver becomes a successful attorney with his own practice, and Peggy becomes the main attraction of a new nightclub show.

Detective Flynn (Alan Bridge) attends the opening of Peggy's new show, and when he visits her backstage, the heartbroken Peggy admits that she confessed to the murder to help Oliver. Flynn then accompanies her to see Oliver, who reunites with her when he learns the truth. Now suspicious that Peter is somehow connected to Brent's death, Peggy sends Oliver to find him at the theater at which he is working as a doorman, while she and Flynn go to search Brent's office one more time. While looking through Brent's desk, Peggy finds Peter's packet of newspaper clippings, which give detailed accounts of how Peter killed two actors, in two separate versions of Hamlet, while playing the lead. Peggy realizes that Peter murdered Brent and rushes to the theater.

Oliver, meanwhile, has been lured into enacting the play with Peter, and is about to be murdered by the obsessed actor when Peggy arrives with the police. After Peter confesses to killing Brent, he is arrested, and Peggy promises Oliver that she will never lie again.

"Cross My Heart" is a remake of Paramount's "True Confession", a 1937 adaptation of Louis Verneuil and Georges Berr's play, which was also written by Claude Binyon, and was directed by Wesley Ruggles, and starred Carole Lombard, Fred MacMurray and John Barrymore.

The working title of this film was "Too Good to Be True", which was also the working title of Paramount's 1947 release "The Trouble with Women", starring Ray Milland and Teresa Wright.

Betty previously appeared with Sonny Tufts in the film "Here Come The Waves" (1944).

Marie Osborne was Betty's stand-in for this film.

Following completion of this film, Betty took a two-week vacation to Mexico City with her friend Eunice MacFarland.

When the script called for Betty to be slapped by Howard Freeman, vaulted over a sofa and meet the 215-pound actor in a slugging match, co-star Sonny Tufts suggested a stunt double. Betty was quoted as saying, "The only time I should get a stunt check is when I'm required to be quiet."

A 1948 Associated Press poll questioned several actors and actresses about their worst film experiences. Betty chose "Cross My Heart" as her worst film, saying, "It stunk from start to finish." Later in her career, Betty couldn't even recall making the film.


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