The story begins in Autumn of 1916, and follows an Irish immigrant named John Lawless (Tommy Steele) as he applies for a butler position with eccentric Philadelphia millionaire Anthony J. Drexel Biddle (Fred MacMurray). Even though the family is a bit strange, Lawless soon learns that he fits right in. Mr. Biddle takes a liking to him immediately. For the rest of the film, Lawless serves as the narrator/commentator.
Mr. Biddle busies himself with his Biddle Boxing and Bible School (located in his stable) and with his alligators in the conservatory. He is also anxious to get America into the War in Europe (World War I), despite the Governments Policy of Neutrality. His wife, Cordelia (Greer Garson), stands quietly by, accepting his eccentricities with a sense of pride and class. Their two sons, Tony and Livingston (Paul Petersen and Eddie Hodges, respectively) are headed off to boarding school, never to be seen in the film again. Their daughter, Cordy (Lesley Ann Warren, in her film debut), is a tomboy with a mean right hook who was educated by private tutors and has had limited contact with conventional society. She's frustrated by her apparent inability to attract suitors and wants to see what's beyond the Biddle manor.
Mr. Biddle reluctantly lets Cordy go to a boarding school as well (after some prodding from both Cordy and from his Aunt Mary (Gladys Cooper), where her roommate teaches her how to lure men with feminine wiles, known as "Bye-Yum Pum Pum". At a social dance hosted by her aunt and uncle, Cordy meets Angier Buchanan Duke (John Davidson, in his film debut) and they fall in love. He tells Cordy that he is fascinated with the new automobile and wants to head to Detroit, Michigan to make his fortune there, instead of taking over his family's tobacco business.
That winter, Cordy comes back to her parents' home and tells them that she is engaged. At first, this is a difficult thing for Mr. Biddle to take. He does not want to give up his little girl. But, after meeting Angie and witnessing first-hand his Jiu Jitsu fighting skills, Mr. Biddle takes a liking to him and accepts the engagement. Then Cordy travels with Angie to New York City to meet his mother (Geraldine Page). Soon the Biddles and the Dukes are making arrangements for a very grand wedding.
Constant condescending comments from Angie's mother are painful for Cordy. To make matters worse, their families' elaborate planning for the "social event of the season" (it is by now Spring of 1917), makes both Cordy and Angie feel pushed aside. The tension reaches a climax when Cordy learns that Angie has abandoned his plans for Detroit, and is instead taking his place in the family business, following his mother's wishes. Cordy angrily calls the wedding off, thinking of Angie as a mama's boy, and Angie storms out of the house. Both families are instantly in a tremendous state of upheaval. Mr. Biddle sends John Lawless to look after Angie.
John finds Angie at the local tavern, contemplating what he will do next. During a rousing song-and-dance sequence, John tries to convince Angie to go back to Cordy. However, Angie is stubborn and thinks of other ways to deal with his problems, among other things saying that he wants to join the Foreign Legion. Angie unwittingly starts a bar fight (with a little help from John) and is hauled off to jail.
The next morning, Mr. Biddle comes to bail Angie out. He tells Angie he has to forget about his own dreams and accept his place in the family business. His words have the desired effect, inspiring Angie to defy his mother and elope with Cordy and go to Detroit. Cordy, however, believes her father talked Angie into it, so to prove his sincerity, amid the cheering of the cell mates, Angie throws Cordy over his shoulder and carries her out of the jail house to start their new life together (Short version ends here). After Mr. and Mrs. Biddle return home a delegation of Marines arrive to inform him he has been made an "provisional captain" in the Marine Corps; and is wanted immediately to go to Parris Island to help/continue training the recruits, now that America is finally entering the War. Mr. Biddle accepts with delight, and the hearty congratulations of his suddenly appearing Bible Boxing Class. Behind the final credits a car is seen driving toward a city skyline (apparently Detroit) dominated by factories spewing smoke to blacken the sky over the city.
- Fred MacMurray as Anthony J. Drexel Biddle
- Greer Garson as Mrs. Biddle
- Gladys Cooper as Aunt Mary
- Tommy Steele as John Lawless
- Geraldine Page as Mrs. Duke
- Hermione Baddeley as Mrs. Worth
- Lesley Ann Warren as Cordelia "Cordy" Biddle
- John Davidson as Angie Duke
- Paul Petersen as Anthony "Tony" Biddle
- Eddie Hodges as Livingston "Liv" Biddle
- Main article: The Happiest Millionaire (soundtrack)