Clocking in at 70 minutes, without a star, and set mostly on a train, The Narrow Margin manages to accomplish a fair amount with minimal resources. The film's protagonist, Sgt. Walter Brown (Charles McGraw), is assigned to protect a mob boss' wife, Mrs. Frankie Neal (Marie Windsor). When Brown and his partner pick up Mrs. Neal, Brown's partner is killed by waiting gangsters before Brown manages to get Mrs. Neal onto a train out of town. And so begins the tense relationship between Brown and Mrs. Neal that plays out on in a tight cabin on a claustrophobic train set.
While he's still reeling from the death of his partner, she tactlessly needles him and he increasingly resents her convenient snitching. Meanwhile, he becomes acquainted with two other passengers - Ann Sinclair (Jacqueline White) and her young son Tommy (Gordon Gebert) - and they threaten to become collateral damage when the mobsters believe that Ann is Mrs. Neal. This further escalates the conflict between Brown and Mrs. Neal as he is forced to protect both Mrs. Neal and Ms. Sinclair. "Sister," Brown tells her, "I've known some pretty hard cases in my time; you make 'em all look like putty. You're not talking about a sack of gumdrops that's gonna be smashed - you're talking about a dame's life! You may think it's a funny idea for a woman with a kid to stop a bullet for you, only I'm not laughing!" "Why shouldn't I take advantage of her - I want to live," Mrs. Neal replies, "If you had to step on someone to get something you wanted real bad, would you think twice about it?"