Connie Ennis (Leigh) lives in New York City and works for a department store as a "comparison shopper"—a real job back in the day, a mild form of corporate espionage in which the comparison shoppers would go into rival stores and pretend to buy products, only to take them back to their own stores to note the price and the quality of the product before returning.
A few days before Christmas, Connie goes into a store called Crowley's and buys a model train from toy department salesman Steve Mason (Mitchum) without even asking about it. When she returns it the next day, he sniffs her out as a comparison shopper. He could turn her in, which would get her blacklisted from other stores and probably fired, but assures her he won't... which then gets him fired after his manager overhears their conversation.
Meanwhile, Connie goes home to her son Timmy. Connie is a war widow, her husband having been killed in World War II. Connie is being courted by handsome, affable lawyer Carl (Wendell Corey). In fact they've been seeing each other for two years, but Connie, still carrying a torch for her husband, doesn't want to commit. Carl is persistent, though, and has just about worn down Connie's resistance when darkly handsome Steve Mason pops back up in her life, returning her comparison shopping packages and later giving Timmy the train set for Christmas.
- Bookends: The first shot is of a train which is revealed to be a model train in a toy store. The last shot is Connie and Steve kissing on the train, which eventually cuts into a shot of a model train in a toy store.
- Dogged Nice Guy: Carl has spent two years hanging out around Connie, taking her out to dinner and bringing presents for Christmas and such, hoping that she'll agree to marry him. Even though Connie says in their first scene together that she doesn't really feel about him that way. Eventually he has an epiphany where he realizes he deserves a girl who does love him.
- Exact Eavesdropping: Little Timmy just so happens to overhear two office women saying that Mr. Crowley of Crowley's department store accepts all returns. Timmy, who has just been blown off by customer service because the train is broken, misunderstands that to mean Mr. Crowley personally accepts returns, so he goes to Crowley's office.
- I Need a Freaking Drink: Christmas Eve has been made awkward by Steve showing up while Carl and Connie are together. Some pained small talk eventually ends with Carl saying "How about a drink?" Steve agrees, and a stressed-out Connie says "I'll get us all one."
- Ironic Name: The snow-bound station that the train pulls into in the opening scene is Palm Beach. That's right when the train is revealed to be a model train.
- Last-Name Basis: Although Connie and Timmy do call each other "Timmy" and "Mom", they also have a habit of referring to each other as "Mr. Ennis" and "Mrs. Ennis". Steve, who is very perceptive and also very blunt, notes the oddness of this and surmises that Connie is looking for a psychological substitute for her dead husband.
- The Lost Lenore: Steve diagnoses this as the reason why Connie is willing to marry Carl despite not really loving him, and why she keeps pushing Steve away despite clearly being attracted to him. She can't get over her dead husband, and is basically still married to a ghost. She even combs Timmy's hair to look like his dad's.
- Meet Cute: Connie buying a train from Steve, returning it, and accidentally getting Steve fired.
- New Year Has Come: Ends on the Midnight Special train to California, Connie and Timmy having boarded, and Connie finding and embracing Steve as revelers celebrate the new year.
- Proscenium Reveal: The opening scene looks like a train going through a snowy forest. Then it looks like a poorly done effect of a train going through a snowy forest. Then it's revealed to be a model train set.
- Romantic False Lead: Carl, who is handsome and gentlemanly and charming and might as well have "Not Robert Mitchum" tattooed on his face.
- Sweater Girl: Janet Leigh was dressed like this often in her career, but maybe never pointier than in this film where she wears some truly shocking bullet bra and tight sweater ensembles. Robert Mitchum revealed that it was because RKO owner Howard Hughes, who had briefly dated Leigh, insisted that she wear "a sweater so tight it made her breasts stand out like traffic cones."