The trio return to Higgins' Wimpole Street laboratory, exhausted from the night's happenings. They talk about the evening and their great success, though Higgins seems rather bored, more concerned with his inability to find slippers. While he talks absentmindedly with Pickering, Eliza slips out, returns with his slippers, and lays them on the floor before him without a word. When he notices them, he thinks that they appeared out of nowhere. Higgins and Pickering begin to speak as if Eliza is not there with them, saying how happy they are that the entire experiment is over, agreeing that it had become rather boring in the last few months. The two of them then leave the room to go to bed. Eliza is clearly hurt ("Eliza's beauty turns murderous," say the stage directions), but Higgins and Pickering are oblivious to her.
Higgins pops back in, once again mystified over what he has done with his slippers, and Eliza promptly flings them in his face. Eliza is mad enough to kill him; she thinks that she is no more important to him than his slippers. At Higgins' retort that she is presumptuous and ungrateful, she answers that no one has treated her badly, but that she is still left confused about what is to happen to her now that the bet has been won. Higgins says that she can always get married or open that flower shop (both of which she eventually does), but she replies by saying that she wishes she had been left where she was before. She goes on to ask whether her clothes belong to her, meaning what can she take away with her without being accused of thievery. Higgins is genuinely hurt, something that does not happen to him often. She returns him a ring he bought for her, but he throws it into the fireplace. After he leaves, she finds it again, but then leaves it on the dessert stand and departs.
If we consider the conventional structure of a romance or fairy tale, the story has really already reached its climax by this point, because Cinderella has been turned into a princess, and the challenge has been met. Then why does the play carry on for another two acts? This would appear completely counter- productive, only if one thinks that this play is only about changing appearances. The fact that the play carries on indicates that there are more transformations in Eliza to be witnessed: this act shows the birth of an independent spirit in the face of Higgins' bullying superiority. The loosely set-up dichotomy between people and objects (i.e., whether Higgins treats people like people or objects) is brought to a head when Eliza flings his slippers in his face, and complains that she means no more to him than his slippers--"You don't care. I know you don't care. You wouldn't care if I was dead. I'm nothing to you--not so much as them slippers." Not only does she object to being treated like an object, she goes on to assert herself by saying that she would never sell herself, like Higgins suggests when he tells her she can go get married. This climactic move forces Higgins to reconsider what a woman can be, and, as he confesses in the final act, marks the beginning of his considering Eliza to be an equal rather than a burden.
One thing to consider in this act is why Shaw has chosen not to portray the climax at the ambassador's party where Eliza can prove how well she has been instructed by Higgins (although his movie screenplay does allow for a scene at the embassy). One reason is that most theatrical productions do not have the capacity to stage an opulent, luxurious ball just for a short scene. But another reason is that Shaw's intention is to rob the story of its romance. We are spared the actual training of Eliza as well as her moment of glory (that is, both the science and the magic); instead, all we get is scenes of her pre- and post- the dramatic climax.