On a cold night in London, patrons leaving the Royal Opera House are trying to find taxis. Eliza Doolittle, a Cockney flower girl, is knocked over by one of them: a young man called Freddy Eynsford-Hill. She admonishes him, becoming even more upset when she sees another man copying down her words. This is Henry Higgins, a distinguished professor of phonetics. Lamenting Eliza’s dreadful accent, he declares that in six months he could turn her into a lady simply by teaching her to speak properly. An older gentleman introduces himself as Colonel Pickering, a linguist who has long studied Indian dialects. As both men have always wanted to meet each other, Higgins invites Pickering to stay with him.
As they leave, the professor distractedly throws some spare change into Eliza’s flower basket. She and her cockney friends wonder what it would be like to live a comfortable life. Eliza’s father, Alfred P. Doolittle, and his drinking companions Harry and Jamie, all dustmen, emerge from a nearby pub. Doolittle, as usual, is searching for money for another drink, and Eliza reluctantly gives him some.
At Higgins’s home the housekeeper, Mrs. Pearce, announces that a young woman has arrived. It is in fact Eliza, who wants Professor Higgins to teach her to speak properly so that she can obtain work in a florist’s shop. Pickering bets Higgins that he will not be able to make good his claim to transform Eliza and even volunteers to pay for Eliza’s lessons. An intensive makeover of Eliza’s speech, manners and dress begins.
Eliza’s father arrives at Higgins’ house the next morning, claiming that Higgins is compromising Eliza’s virtue. Higgins is impressed by the man’s natural gift for language and his brazen lack of moral values. He and Doolittle agree that Eliza can continue to take lessons and live at Higgins’ house if Higgins gives Doolittle five pounds for a drinking spree. While Eliza endures the long and difficult speech tutoring, the servants lament the long hours that Higgins imposes on the entire household. Just as they are all about to give up, Higgins eloquently speaks of the glory of the English language and Eliza makes the long-awaited breakthrough.
For her first public tryout, Higgins takes Eliza to his mother’s box at Ascot Racecourse. Eliza initially impresses with her polite manners but then unintentionally shocks everyone when she excitedly reverts to Cockney during a horse race. But she has captured the heart of Freddy Eynsford-Hill, the young man who knocked her over outside the Royal Opera. Freddy calls on Eliza, but after the Ascot disaster she refuses to see anyone. He declares that he will wait for her as long as is necessary.
After further preparation Eliza is finally ready for an even more difficult test: the Embassy Ball. Higgins, his mother and Colonel Pickering are all nervous as to how the evening will unfold. But Eliza passes the test brilliantly. Everyone at the ball is fascinated by her, including a Hungarian phonetician named Zoltan Karpathy. Higgins’ triumph is complete when the Queen of Transylvania not only notices Eliza but encourages her son, the Crown Prince, to dance with her.
After the ball, Pickering flatters Higgins about his triumph, while the professor expresses his pleasure that the experiment is finally over. The episode leaves Eliza feeling used and abandoned, particularly as Higgins completely ignores her except to ask where he has left his slippers. When Eliza throws them at him, Higgins is completely mystified by her ingratitude.
Deciding to leave the house that very night, Eliza finds Freddy still waiting outside. He is overjoyed to see her, but Eliza cuts him off, telling him that if he really loved her, he would show her rather than talk about it. Returning to Covent Garden, Eliza’s old friends no longer recognize her. But her father, surprisingly dressed in top hat and tails, does. He explains bitterly that as a result of Professor Higgins’ intervention, he has received a surprise bequest from an American millionaire, which has ruined him by raising him to middle-class respectability. The worst thing is that he now has to marry the woman he has been living with for all these years. Doolittle and his friends decide to have one last drinking spree before his wedding the next morning.
Higgins and Pickering are upset to discover that Eliza has left, and Pickering leaves to try and find her. Concluding that men are far superior to women in everything, Higgins nevertheless seeks his mother’s advice. He is astonished to find Eliza having tea with her. Higgins demands that she return home, but Eliza accuses him of wanting her back only to fetch and carry for him. She further declares that she was foolish to ever think that she needed Higgins, and that she will marry Freddy instead. The professor is struck by Eliza’s spirit and independence and asks her once more to stay with him, but she tells him that he will not be seeing her again.
As Higgins returns home alone, he begins to discover what his real feelings for Eliza might be. As he listens once again to the first recording he made of Eliza’s voice, the recording is suddenly replaced by Eliza’s real voice. Without even looking up, Higgins asks her if she has any idea where his slippers might be…