Polly Parish (Rogers) is a young single woman working as a salesclerk for John B. Merlin and Son, a large department store in New York City. Unfortunately for Polly, she was hired as a seasonal employee, and on Christmas Eve she is laid off due to the holiday shopping season being over. After leaving Merlin's, she is walking home when she sees a woman dropping off a baby at the doorstop to an orphanage. Polly stops to catch the baby when it looks like he's about to roll off the step—which is exactly when the door opens.
The orphanage assumes that Polly is the baby's mother, despite her denials. They contact her last place of employment, and David Merlin (Niven), the "Son" of John B. Merlin and Son, assumes that Polly abandoned her baby because she lost her job. So David offers Polly her job back, but only if she takes back her baby. Hilarity Ensues, as well as romantic sparks between David and Polly.
Bachelor Mother was a big film for both its stars. Ginger Rogers was breaking out as a headliner after years spent in musicals and as Fred Astaire's partner in dance movies. It was one of the first starring roles period for David Niven, although his career as a leading man was soon put on hiatus by World War II.
- Acme Products: The employment agency that got Polly the job at Merlin's, to whence she returns after getting laid off, is called Acme Employment Services.
- As Long as It Sounds Foreign: The faux-Swedish gibberish that Polly and David speak to each other after David introduces Polly to his friends as a Swede who doesn't speak English.
- Cassandra Truth: The whole movie. Nobody—the orphanage folks, David, Polly's landlady, Polly's would-be boyfriend Freddie—believes Polly when she insists that the baby isn't hers. Eventually she gives up, and, naturally, she winds up bonding with the baby boy.
- Comically Missing the Point: David either doesn't pick up on or deliberately ignores his father's irritation at David's raucous lifestyle.John Merlin: Did you sleep in jail?
David: No I didn't sleep at all. I had to wait for the court to open.
- Doorstop Baby: Literally, in the case of the baby left on the doorstop of the orphanage, whom Polly becomes entangled with after she stops to keep him from falling off. She eventually names him John.
- Hypocritical Humor: In the opening scene John Merlin addresses all of his employees over the PA system, wishing them a merry Christmas. Then an underling goes around dropping off dismissal notices for all the seasonal employees.
- Insignia Ripoff Ritual: Freddie gets promoted from stockboy to assistant floorwalker at Merlin's, a job which requires a carnation in the lapel of his suit. When he winds up nabbing none other than David Merlin for shoplifting, he gets demoted back to stockboy, and the supervisor rips his carnation out of his lapel.
- King Incognito: David breaks the toy duck Polly got for John, and tells her to exchange it at Merlin's. She tells him that she can't, because Merlin's won't take exchanges without a receipt. He insists that they will, and she says they will for him because he's the boss's son. David then says he'll prove it by returning the duck while in disguise. He fails spectacularly, and he's on the verge of getting arrested for shoplifting when the security guard realizes who he is.
- Language Fluency Denial: Polly is nervous when David issues a last-minute invitation to be his date for a high-society dance. He thus introduces her to his friends as a woman from Sweden who doesn't speak English. They speak faux-Swedish gibberish to each other throughout the evening.
- Lawful Stupid: David has busted out a book on how to take care of babies. The instructions say that to feed the baby, you should take the food and rub it into the baby's stomach. David insists to Polly that she should rub the food on the baby's stomach because dammit, the man writing the book knows what he's talking about. Polly discovers that the pages of the book are stuck together and the rub-on-stomach business is about using baby oil to relieve gas.
- Meet Cute: Boss calls underling into his office and gives her a permanent job so she can keep her baby.
- New Year Has Come: David and Polly go to a fancy society dance for New Year's, and then make their way to Times Square for the ball drop (maybe it wasn't all tourists in 1939).
- Not What It Looks Like: When David goes to court Polly, his father, who is following, thinks David is visiting his mistress and illegitimate son.