At the reading of the will of young Patrick Dennis’s (Kirby Furlong) father, a trustee, Mr. Babcock (John McGiver), reveals 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). Taking a train to New York City (“Main Title Including St. Bridget”), Agnes and the boy arrive at Mame’s home, where they walk into a wild party that Mame is giving for a holiday she herself created (“It’s Today”). Patrick asks if he may slide down her banister, then reveals his true identity. Mame introduces the boy to her friends, including a renowned stage actress (and famous lush), Vera Charles (Beatrice Arthur).
Mame decides that she wants to fill the child’s life with adventure (“Open a New Window”). She enrolls him in “the School of Life”, a very non-traditional school. But when Vera inadvertently leads Babcock to the school, Patrick is taken from Mame’s custody. Simultaneously, the stock market crash leaves Mame penniless. Vera offers her a small role as “The Man in the Moon” in her newest operetta about a lady astronomer. Mame flubs her one line and causes the play to be a disaster, which puts a major rift in their friendship. Patrick, who was in the audience, reassures Mame that he still loves her (“My Best Girl”).
Mame works a string of jobs. One is in a department store, where she meets Beauregard Jackson Pickett Burnside (Robert Preston), to whom she attempts to sell a pair of roller skates. She cannot write up a cash order and is fired. Unable to remove the skates she has demonstrated, Mame roller skates home, dejected due to her inability to pay manservant Ito (George Chiang) or Agnes, who have loyally stayed with her. Mame decides to lift everyone’s spirits by decorating the house for Christmas and giving everyone their Christmas gifts early (“We Need a Little Christmas”), which include Patrick’s first pair of long pants. Agnes and Ito surprise her with the news that the butcher bill has been paid.
Beau, who has been looking for Mame since she was fired, appears at her front door and invites everyone to dinner. Beau falls in love and brings Mame and Patrick to his family’s plantation in Peckerwood, Georgia, where they’re greeted coolly by Sally Cato (Joyce Van Patten). A number of Beau’s relatives, especially Mother Burnside (Lucille Benson) and Cousin Fan (Ruth McDevitt), are unhappy about Beau marrying a “Yankee”. Sally decides to invite Mame to a foxhunt. Despite not knowing a thing about riding a horse, Mame accepts. After a wild ride, Mame accidentally captures the fox. All of Beau’s family and friends (except for Sally) now sing her praises (“Mame”).
She and Beau go on an extended honeymoon, traveling all over the world (“Loving You”). While they’re away, 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 Beau dies in an avalanche in the Alps, Mame returns home to be reunited with a now-grown Patrick, who is dating a snobby, conservative girl named Gloria Upson (Doria Cook-Nelson).
When Mame meets Vera for a drink, the two trade 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 reminisce about men 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 suckers are starving to death.” Six months later, Agnes returns home, visibly pregnant. Agnes then describes what she did while living it up (“Gooch’s Song”).
Mame visits the Upsons (Don Porter and Audrey Christie) at their home, Upson Downs in Connecticut. She learns for the first time that Patrick and Gloria are engaged. After spending several hours with the Upsons and finding them insufferable bores and bigots, Mame is asked 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”. Afterward, she is candid with Patrick about her disdain for the family. He admits that he’s ashamed of her and her “crazy” friends. A heartbroken Mame wonders what she did wrong with this boy she raised (“If He Walked Into My Life”).
Mame and Patrick apologize to each other at her home. They are dressed for company: the Upsons. Mame promises to behave and Patrick meets Mame’s new maid, Pegeen (Bobbi Jordan). Mr. and Mrs. Upson announce that the property they’d wanted has been bought by some “Jew lawyer”. Vera and several men suddenly barge into Mame’s house, singing “It’s Today” (the reprise). Vera toasts the new couple, mistaking Pegeen for Gloria. At that moment, Agnes comes downstairs, and the Upsons discover she is going to be an unwed mother. A busload of other unwed pregnant women arrives, singing an “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 us.” Patrick, visibly upset, also leaves.
Years later, following World War II, 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”).