This is the sort of movie where a character's behavior is best explained by saying "the plot requires it." A Warner Brothers entry at the height of the film-noir era, "Nora Prentiss" was a box-office success and a boost for Ann Sheridan's career. Today, however, it looks like a parody of its own genre. The main problem is the script, which director Vincent Sherman and others adapted from a short story by Paul Webster. The unlikely tale proceeds through a series of melodramatic twists and turns involving a married San Francisco physician named Dr. Richard Talbot (Kent Smith) and the title character, a stunning nightclub singer (Sheridan). Melodrama, of course, always under girds film noir, but when audiences watch this kind of movie they want to believeif only for 90 minutesthat the events could really be happening. With this picture, it's a tough job. Making the situation worse is dialog that strains to sound memorable and ends up sounding phony. There are warning signs from the start, with the first five minutes being so heavy with exposition that you want to shout, "Enough! I get the point!" Still, there are some good things to be said about "Nora Prentiss." The central plot gimmick is an interesting one, despite its implausibility. James Wong Howe's cinematography effectively captures the story's shadowy moods. And the alluring Sheridan, as usual, gives a solid performance. But the casting of Kent Smith presents problems. During Smith's time with Warners the studio liked to assign him the role of an earnest wimp involved with a captivating woman (see him opposite Joan Crawford in "The Damned Don't Cry," which Sherman directed three years later). So it's easy to see how producers chose this actor to play the conservative doctor. But Smith lacked charisma, and in this film the audience has to keep reminding itself that the vibrant Nora finds the dull Dr. Richard Talbot fascinating. Why does she, anyway? (The plot requires it.) Why does this worldly but essentially decent woman take up with a repressed family man in the first place? (The plot requires it.) And why does Richard veer off into a tangle of unbelievable actions that grow increasingly grotesque as the film rolls toward its disastrous finale? (The plotwell, you get the idea.) For the rest, there are competent performances by Rosemary DeCamp as Richard's wife, by Bruce Bennett as Richard's partner in a medical practice, and by Robert Alda as Nora's nightclub boss and lover-in-waiting. The musical score is by Franz Waxman.