Impoverished British bank clerk, John Ross (Jack Watling) is hopelessly in love with drop-dead gorgeous Irene James (Pat Kirkwood). Ross will do anything to win Irene's affections including embezzlement. She ends up marrying him, but she can't give up her true love, slimy counterfeiter Jimmy Smart (Sydney Tafler). By the time Ross realises that his former fiancee Vera Lamb (Joy Shelton) was the right girl all along, it's a shade too late. The magnificent Thora Hird steals the show in 'Once a Sinner' as the fickle Irene's slatternly mother.