The play recounts the long, rainy afternoon that Hally ("Master Harold") spends with Sam and Willie, two middle-aged African servants of his parents' household, in a tea shop owned by Hally's mother. Sam and Willie have cared for seventeen-year-old Hally his whole life.
At the start of the play Sam and Willie are practicing ballroom steps in preparation for a major competition, while maintaining the tea shop. Business is slow due to the rain. Sam is quickly revealed as being the more worldly of the two. When Willie, in broken English, describes his ballroom partner and girlfriend as lacking enthusiasm, Sam correctly diagnoses the problem: Willie beats her if she doesn't know the steps.
Hally then arrives home from school, and cheerfully asks if the two are ready for the competition. Sam is the unacknowledged yet de facto mentor to the boy since childhood, and has always treated Hally as his nephew/ward. Sam hopes to skillfully guide Hally through the difficult passage from childhood into manhood. And hopefully thereby, takes up those values and viewpoints of an adult; and a man, that Sam, has over the years lain down; and, that all men try to leave as their legacy of shepherding an adolescent boy into manhood. Willie, for his part, has always played the "loyal black"; who has always called the white Afrikaner boy (now young man) "Master Harold"; As if, as a man of forty-five, he was addressing a superior; even when Hally was six.
The conversation between the three moves from Hally's school-work, to an intellectual discussion on "A Man of Magnitude", where they mention various historical figures of the time and their contribution to society, to flashbacks of Hally, Sam and Willie when they lived in a Boarding House. Hally warmly remembers the simple act of flying a kite Sam had made for him out of junk; we later learn that Sam made it to cheer Hally up after he was embarrassed greatly by his father's public and continuing drunkenness. Conversation then turns to Hally's 500-word English composition. The play reaches an emotional apex as the beauty of the ballroom dancing floor ("a world without collisions") is used as a transcendent metaphor for life.
Almost immediately despair returns: Sam had early on mentioned why Hally's mother is not present; the hospital had called about his father, who has been there receiving treatment for complications from a leg he lost in World War II, to discharge him, and she had left to bring him home. However, Hally, indicating that his father had been in considerable pain the previous day, insisted that his father wasn't well enough to be discharged, and that the call must've been about a bad turn, rather than a discharge notice. A call from Hally's mother at the hospital confirms that Hally's father, is manipulating the hospital into discharging him, although he is indeed, not feeling any better than before, so it's still unofficial, and Hally remains hopeful that the discharge won't happen. A second call from Hally's mother later reveals that the discharge is official, and Hally's father is now home.
Hally is distraught about this news, since his father, who in addition to being crippled, is revealed to be a tyrannical alcoholic, and his being home will make home life unbearable with his drinking, fighting, and need for constant treatment, which includes demeaning tasks of having to massage his stump, and empty chamber pots of urine. Hally vents to his two black friends years of anger, and pain, viciously mocking his father and his condition. But when Sam chastises him for doing so, Hally, although ashamed of himself, turns on him, unleashing vicarious racism, that he learned from his father, creating possibly permanent rifts in his relationship with both Sam and Willie. For the first time, apart from hints throughout the play, Hally begins explicitly to treat Sam and Willie as subservient help rather than as friends or playmates, insisting that Sam call him "Master Harold" and spitting on him, among other things. Sam is hurt and angry and both he and Willie are just short of attacking Hally, but they both understand that Hally is really causing himself the most pain.
There is a glimmer of hope for reconciliation at the end, when Sam addresses Hally by his nickname again and asks to start over the next day, harkening back to the simple days of the kite. Hally, horrified about what he's done, is barely able to face Sam, responding without looking up "It's still raining, Sam. You can't fly kites on rainy days, remember," then asks Willie to lock up the tea shop, and walks out into the rain, as Sam mentions that the bench Hally sat on as he flew the kite said "Whites Only" but Hally was too excited to notice it, and that he can (figuratively) leave it at any time. The play ends while Sam and Willie console each other by ballroom dancing together.