The film starts with the reading of the will of Patrick Dennis' (Kirby Furlong) late father, by his trustee, Mr. Babcock (John McGiver). The will states that Patrick is to be left in the care of his aunt, Mame Dennis (Lucille Ball), as well as his nanny, Agnes Gooch (Jane Connell). The two take a train ride to live with Mame (Main Title Including St. Bridget). When they arrive a day early, they walk into a big party that Mame is giving for a holiday she herself created. (It's Today) Patrick introduces himself by asking if he may slide down her bannister, then reveals that he is Patrick. Mame introduces him to several of her friends, including aspiring stage actress and famous lush, Vera Charles (Bea Arthur).
The following morning, Patrick awakens a hungover Mame with his bugle. After Patrick tells Mame what Mr. Babcock has said about her, she decides that she wants to fill his life with adventure (Open A New Window). She decides to enroll him in "the School Of Life," a very non-traditional school, but when Vera Charles inadvertently leads the trustee, Mr. Babcock, to Patrick's school, Patrick is taken from Mame's custody. In that same moment, Mame gets a phone call and learns that the stock market crash has left her without any money to hire a lawyer to get custody of Patrick back. Vera, knowing that Mame is now in need of money, offers Mame a very small role as The Man In The Moon in her newest operetta about a lady astronomer. Unfortunately, Mame flubs her one line and causes the play to be a disaster, which puts a major rift in her friendship with Vera. Meanwhile, Patrick, who was in the audience, reassures Mame that she's not a failure and lets her know that he still loves her (My Best Girl).
Now broke, Mame has worked a string of jobs, including one selling shoes. While working in the shoe section of the department store, a customer comes in wanting a present to send to someone back home. Mame helps him make the decision to buy a pair of roller skates by trying them on. The customer tells her of his name - Beauregard Jackson Pickett Burnside (played by Robert Preston). However, Mame's inability to write up a cash order as opposed to a C.O.D. order gets her fired. Mame roller skates home, dejected because she's unable to pay Ito (George Chiang) and Agnes, who reassure her that they're not going anywhere. Even though it's only a week from Thanksgiving, Mame decides to lift everyone's spirits by decorating the house for Christmas and giving everyone their Christmas gifts (We Need A Little Christmas), which include Patrick's first pair of long pants. Agnes and Ito surprise Mame with the news that the butcher bill has been paid. Mame promises to pay them back someday. Meanwhile, Beau, who's been looking for Mame since she was fired earlier that day, finally finds Mame's house and invites everyone to dinner, and it's obvious that the two are meant for each other. (She says to Agnes that he looks like Rhett Butler, however neither the movie or the book had been released yet. 1939 and 1936 respectively)
Beau brings Mame and Patrick to his plantation in Peckerwood, Georgia where they're immediately greeted by Sally Cato (Joyce Van Patten). However, much of Beau's family, especially Mother Burnside (Lucille Benson) and Cousin Fan (Ruth McDevitt), are not happy about Beau marrying a "Yankee". Sally then invites Mame to a foxhunt. Despite not knowing a thing about horseriding, Mame accepts the invitation. The following day, Mame accidentally wins the fox hunt, despite not knowing what she was doing, and all of Beau's family and friends, except for Sally, sing the praises for Mame.
Mame and Beau, now happily married, go on an extended honeymoon, traveling all over the world (Loving You). Meanwhile, Patrick goes from a young child who pulls in a B+ average to a high school senior (Bruce Davison) flunking many classes (The Letter). When an avalanche in the Alps kills Beau, Mame returns home and is reunited with a now grown Patrick, who is dating a very snobby conservative girl named Gloria Upson (Doria Cook-Nelson). Mame, who decides that she's tired of looking like she's just come from a funeral, goes to reunite with her old friend, Vera, for a drink. The two enjoy some drinks and some snippy comments, which they insist are not being made out of hatred, but simple honesty, as that's what Bosom Buddies do. The two come home and continue to reminisce and discuss men that they've dated. Agnes, who is listening to the conversation, admits that she's never had a date. Mame and Vera decide to give the uptight, frumpy Agnes a makeover and send her out to live, because "Life is a banquet, and most poor sons-of-bitches are starving to death." After Agnes comes out of her bath with her new image, she goes off in a taxicab.
Six months later, Agnes returns home, visibly pregnant. At the same time, Mame is currently visiting with her guests, Patrick and Gloria, and they agree to bring Gloria's parents to Mame's home to meet. However, once Patrick sees Agnes, who's hiding in the kitchen, he decides it'd be a better idea for Mame to visit the Upsons at their home, since Patrick is ashamed to have the Upsons see an unwed, pregnant Agnes. Agnes then describes what she did after her big makeover (Gooch's Song).
Mame visits the Upsons (Don Porter and Audrey Christie) at their home, Upson Downs. She learns there that Patrick and Gloria are engaged. After spending several hours with the Upsons, Mame discovers that she definitely dislikes the family and their overly conservative and bigoted views on everything from African-Americans and onward. (They praise their African-American maid, noting that "so many of them are so snotty these days" and ask Mame to help pay for a piece of property next door to Upson Downs so that Patrick and Gloria could live there, as opposed to "the wrong kind of people.") When Mame leaves, she confronts Patrick about her disdain for the family, calling him a snob when he admits that he's ashamed of her and her "crazy" friends. A heartbroken Mame drives home, wondering what she did wrong when he was younger (If He Walked Into My Life).
Mame and Patrick apologize to each other off-screen and are dressed for company - the Upsons. Mame promises to behave. Patrick, still embarrassed by Agnes's condition, begs Agnes to stay in her room while the Upsons are there, while Mame reminds her to take her calcium pills. Patrick talks to Mame's new maid, Pegeen (Bobbie Jordan), for a moment before the Upsons arrive. After arriving, Mr. and Mrs. Upson announce to Mame that the property they'd wanted had been bought, complaining about being outbid by "some Jew lawyer". Suddenly, Vera and several men barge into Mame's house, singing (It's Today reprise). Vera toasts to the new couple, mistaking Pegeen for Gloria. At that moment, Agnes comes downstairs because her calcium pills are in the kitchen. Mame invites her to sit with everyone. When Mrs. Upson asks Agnes what Mr. Gooch does, she says "My father's passed away." When Mrs. Upson states that she meant her husband, Agnes declares that she's unwed and that her baby's going to be a little bastard (although in the film, she's cut off after "ba ..."). Suddenly, a large group of unwed pregnant women barge in, singing. (Open A New Window reprise) Mame reveals to the Upsons that she bought the property next door so she could build the Beauregarde Burnside Memorial Home For Single Mothers. This is the final straw, and the Upsons leave, angry that Mame isn't "one of them." Patrick, visibly upset, leaves the house.
Years later, Patrick and Pegeen are married and have a child, Peter. Mame, who is going on a trip to Siberia, requests that Peter be allowed to go with her. Although Patrick and Pegeen resist at first, once Peter quotes Mame's "life is a banquet" line, they relent. The two get onto a plane, and Patrick states that Mame has not changed and that she's "the Pied Piper." Mame and Peter wave goodbye and go into the plane. The plane takes off, followed by clips of Mame embracing Vera, Agnes, Beau, adult Patrick, and young Patrick (Finale: Open A New Window/Mame).