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Henry Higgins, an arrogant, irascible professor of phonetics, boasts to fellow linguist Colonel Pickering that he can train any woman to speak so properly that he could pass her off as a duchess, including Eliza Doolittle, a poor girl with a strong Cockney accent whom he encounters selling flowers in Covent Garden. (In the terms currently used by linguists, and which did not yet exist in the period of the show, Higgins proposed to take a speaker of basilect and teach her to speak acrolect.) Pickering is intrigued by Higgins's boast and wagers that he cannot make good on his claim. Higgins takes on the challenge and begins an intensive make-over of Eliza's speech, manners and dress in preparation for her appearance at the Embassy Ball.

Complicating matters is Eliza's father, Alfred P. Doolittle (Stanley Holloway), a cheerfully amoral and drink-loving dustman, who shows up to extract money from Higgins for compromising Eliza's virtue. Higgins is impressed by the man's natural gift for language and his brazen lack of moral values ("Can't afford 'em!") and flippantly recommends Doolittle to an American millionaire who is seeking a lecturer on moral values. In the end, Doolittle gets a surprise bequest of four thousand pounds a year from the millionaire, raising him uncomfortably into middle-class respectability.

Meanwhile, Eliza endures speech therapy, endlessly repeating phrases such as "In Hertford, Hereford and Hampshire, hurricanes hardly ever happen” (to demonstrate that "h"s must be aspirated) and "The rain in Spain stays mainly in the plain" (to emphasize the "a"). Just as things seem hopeless, she suddenly "gets it" after Higgins eloquently speaks of the glory of the English language, and thereafter her speech is transformed into an impeccable upper class English accent. For her first public tryout, Higgins takes her to Ascot Racecourse, where she makes a good impression with her polite manners but shocks everyone by her vulgar Cockney attitudes and slang (thus establishing one of the show's themes, that good elocution is only "skin deep.") However, she still captures the heart of an eager young man named Freddy Eynsford-Hill.

The final test hinges on Eliza's passing as a lady at the Embassy Ball, which she does successfully, despite the presence of a Hungarian phonetics expert who seeks to unmask her identity. After the ball, Higgins's ungrateful boasting of his triumph and his pleasure that the experiment is now over leave Eliza feeling used and abandoned. She walks out on him, leaving the seemingly clueless Higgins mystified by her ingratitude. But Higgins soon realizes his feelings for her--that he has "grown accustomed to her face." When Eliza tentatively returns to him, the musical ends on an ambiguous moment of possible reconciliation between teacher and pupil.
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