Gail Preston (Rita Hayworth) is the star at the Swing Swing club, beautiful but with a talent for creating enemies. On one particular night, she bawls out Charles Waverly (John Gallaudet), the master of ceremonies, then undermines the club owner's (John Spacey) authority by threatening to quit unless he does as she says. She has him fire the bandleader, Swing Traynor (Robert Paige), for falling in love with her sister, Ann (Wyn Cahoon), instead of her. With her temper, Gail requires a bodyguard, Frank (Marc Lawrence), whom she also fights with. Then there is her thieving business manager, Jules Stevens (Arthur Loft). She threatens to go to the police about missing assets. Sensing trouble on such an active night, Gail summons Inspector Kellogg (Don Terry) to the club for protection. Kellogg arrives too late. Gail is killed at the end of her performance. Thereafter, a man admits to the crime, then commits suicide. It was Gail's estranged husband, whom she supported for years before he was cut off. But it turns out his gun does not match the bullet found in Miss Preston. Someone else meant to kill her, and succeeded. Several of the suspects are after Gail's diary for various reasons, where she had written everyone's secrets. Kellogg gets them to reveal what they are hiding. Most are involved in illegal doings, but nothing concrete as to the murderer. With only suspicions, Kellogg arrests Traynor. If he is innocent, Kellogg hopes it will bring forth the real criminal. The break in the case comes when Kellogg discovers the murder weapon at the Swing Swing club. The gun was attached to a spotlight and triggered by a wire and a spinning color wheel. Kellogg has the crime reenacted entirely, with one change. The colored light shines on the suspects instead of the singer. Thinking the gun is still there, the killer panics, fearing it will go off again. The plan works and the question of the title is answered.

Rita was featured in seductive gowns and jet black hair in Who Killed Gail Preston?, to copy the Hedy Lamarr look from Algiers. She was perfectly contemptible in her role. Unfortunately, she is killed off about a third of the way through the film.