The film opens at J.B. Merlin & Son's department store on the Upper East Side of Manhattan. Polly Parish (Reynolds) works in the Millinery Department, and she is summoned to the office of the store manager where she is fired. The manager informs her that she is overselling hats, which creates too many returns and too much work for the accounting department. After work, Polly walks home and wonders what she will do to make ends meet.
On a step, she encounters an abandoned baby in a blanket and instinctively picks it up to comfort it. As she bends over, the door behind the step opens and a woman ushers Polly and the baby inside. Polly has not noticed the sign that indicates the door belongs to an orphanage. When she denies the baby is hers inside, the staff disbelieve her, having experienced countless women ashamed to admit that they bore a child out of wedlock. Polly explains that she has just lost her job at J.B Merlin & Son's and insists there is no way that she will care for a baby that is not hers.
After she leaves, the head of the orphanage decides to intervene on her behalf, knowing the Merlins to be charitable. He convinces the firm to hire Polly back, and she is summoned to a meeting with Dan Merlin (Fisher) the son of the titular owner. Dan informs her that she will be hired back at $10 more per week and that a gift will be delivered to her apartment later that night. When the baby arrives, Polly is flabbergasted. She convinces a friend to help her deliver the baby back to Dan at his home on East 63rd Street.
They leave the baby in the care of Dan's butler, whom Dan enlists to help him return the baby to Polly. They track her down to a dance hall where she is hoping to win a prize. Eddie gets into a scuffle and gets thrown out. So he goes to Polly's apartment and waits for her there with the baby. Forced to care for the baby, Polly makes do and grows fond of him. She names the child John, and Dan checks in on her from time to time.
On New Year's Eve, Dan is stood up by his date, because he had forgotten to call her earlier. He arranges for Polly's landlady to watch John while they go out together. He takes her to the department store to get an outfit for a night out on the town. When he drops her off back at home, he jokes that she is fired so that he can kiss her.
Meanwhile, J.B. Merlin has been misinformed that he is a grandfather, mistaking John to be Dan's son. He begins to make arrangements for Dan to have full custody of the child. Polly panics at the thought of losing John. So she pretends that her landlady's son is John's real father. The ensuing confusion leads to a full confession of love from Dan and a happy union for the new family.Eddie Fisher as Dan Merlin
Debbie Reynolds as Polly Parish
Adolphe Menjou as J. B. Merlin, Dan's father and Polly's ultimate boss
Tommy Noonan as Freddie Miller
Nita Talbot as Mary
Una Merkel as Mrs. Dugan
Melville Cooper as Adams
Gil Stratton as Mike Clancy
Bill Goodwin as Mr. Creely
Howard McNear as Mr. Appleby
Robert H. Harris as Mr. Hargraves
Mary Treen as Matron
Edward Brophy as Dance Contest Judge (as Edward S. Brophy)
Scott Douglas as Bill Rand
Music by Josef Myrow and lyrics by Mack Gordon;Bundle of Joy
All About Love
Some Day Soon
I Never Felt This Way Before
Worry About Tomorrow
Lullaby in Blue
The original narrative of Bundle of Joy was produced by Joe Pasternak who had developed a winning formula in Universal Picture's Berlin division. His second picture with Henry Koster (director), Felix Jackson (writer), and Franciska Gaal (actress) was Kleine Mutti (Little Mother, 1935) about the orphan Marie who raises a foundling and ends up marrying a banker. The German-language film was remade in English as Bachelor Mother three years later. Bundle of Joy is a musical adaptation of Bachelor Mother, and Jackson retains story credit on both pictures.
The movie's premiere was given at the Capitol Theatre in New York on December 19, 1956. The event doubled as a fundraiser for refugees of the Hungarian Revolution, which had ended only a month earlier. George Jessel was the master of ceremonies. Archduke Leopold of Hapsburg and his sister were notable guests along with other celebrities and executives of RKO pictures. The event raised $21,000 for CARE and First Aid for Hungary, which was formed on October 29, 1956 in order to "bring aid to the hundreds of Hungarian Freedom Fighters who had been forced to seek refuge in neighboring Austria and had thus cast an enormous burden upon that country".
Carrie Fisher tells the story in her memoir and subsequent documentary film Wishful Drinking (2010) that Reynolds was pregnant with her during production of this film as well as Tammy and the Bachelor.
During production, director Norman Taurog was in the early stages of Alzheimer's disease. Very little was known about the illness at the time, so Debbie Reynolds and the rest of the cast and crew "just coped with" his unexplained memory losses and constant repeated instructions.