Millie Dillmount arrives in Manhattan in 1922 after leaving her home in rural Kansas. She has two goals in mind; to become a “modern” woman and to marry a rich man. These two goals propel Millie’s story and act as a primary plot motivator.
The excitement of Thoroughly Modern Millie begins when a naive Millie is almost immediately mugged once she arrives in New York, so she intentionally trips a young man, Jimmy Smith, to get his attention. Jimmy does not offer any help to Millie, but he does offer her some advice. He tells her to check into the Hotel Priscilla, which is known to serve young women, largely wanna-be actresses.
Meanwhile, the young, female residents at the Hotel Priscilla are shocked by the front page story in the daily newspaper, "White Slavery." One of the young women, Ethel, tells the other girls that there are rumors that orphans have been disappearing. Mrs. Meers, the hotel owner, enters and delivers Ethel a telegram. Her relative has died and she is officially an orphan. This exchange foreshadows the dark theme of being sold into sex slavery, which recurs throughout the show.
Millie falls behind on her rent and Mrs. Meers is about to kick her out, when she meets Dorothy Brown, a rich, young woman who would like to experience the way poor people live. Mrs. Meers eventually lets Millie stay, and gives Dorothy Ethel’s old room. When Millie asks about Ethel moving out, Mrs. Meers tells Millie that Ethel has been offered an acting job in "the Orient." Meanwhile, Mrs. Meers begins hatching a plan to kidnap and sell Dorothy. This again touches on the dark theme of sex slavery that runs throughout the show and, of course, its predatory nature.
That night, Millie and the girls from the hotel go out looking for a speakeasy. Millie ends up running into Jimmy, and he helps her into a club. Jimmy and Millie dance until the club is busted by the police. Jimmy and Millie end up spending time together in a jail cell, and Jimmy realizes he has feelings for Millie. Millie is so driven by her desire to marry a rich man that she does not consider her feelings for Jimmy. In fact, Millie believes Jimmy is so poor that she even lied and told him she is engaged to her boss.
Jimmy invites Millie to a fancy event and she agrees to accompany him. They meet the enchanting performer, Muzzy, who acts as a source of guidance throughout the show. Jimmy and Millie kiss at the end of the night, but later that same night, Millie sees Jimmy sneaking out of Dorothy's room.
Though Millie is hurt and still has feelings for Jimmy, she has decided she will move on. She starts pursuing her boss, Graydon, but he eventually falls for Dorothy instead. Jimmy and Millie finally speak and he confesses that he was simply asking Dorothy for advice about his feelings for Millie, so she agrees to go to dinner with him.That evening, Jimmy professes his love for Millie, but Millie still has her sights set on marrying a rich man. Millie talks to Muzzy, who explains that she herself married for love rather than money and has not regretted it a day in her life.
After speaking to Muzzy, Millie runs into Graydon who is upset that Dorothy has stood him up for their date and has apparently checked out of the Hotel Priscilla. Millie immediately realizes something doesn't add up and concocts a plan.
With the help of Muzzy disguised as a vulnerable orphan, they catch Mrs. Meers in the act of trying to sell women into white slavery. They busted Mrs. Meers just in time and were able to rescue Dorothy.
Finally, Jimmy proposes to Millie, who enthusiastically accepts. She has finally decided that she values love more than money. It is only then that she learns both Jimmy and Dorothy are wealthy heirs to Muzzy’s fortune. Throughout the show, Millie is so fixated on marry a rich man, that she almost throws away both love and money. She had to make the decision that she would rather be poor and in love than married to a rich man that she didn’t care about, it was only then that Jimmy revealed his true status.
