In Alabama in the 1880s, the wealthy Keller family has just given birth to a baby girl, Helen Keller. Captain Arthur Keller and his second wife, Kate Keller, summon a doctor to treat Helen for a fever. Soon after, they learn that Helen has lost her ability to see or hear.
Five years pass, and Helen is now a little girl. She can’t communicate with anyone, and so she spends her days horsing around, misbehaving, and sometimes attacking other children. Her family does very little to change her behavior—in fact, Kate sometimes rewards Helen with candy even when she’s been bad. Helen’s half-brother, James Keller (from Arthur’s first marriage) is cynical about Helen’s chances of ever learning how to read or write, and suggests that Arthur send Helen to an asylum.
Even though the Kellers have summoned dozens of doctors to teach Helen, Kate remains cautiously optimistic. She reaches out to a young woman named Annie Sullivan. Annie Sullivan, the audience learns, grew up in Massachusetts. She and her beloved brother, James Sullivan, had a rough early life, and spent years in an almshouse. When Annie was still young, James died, and Annie seems to blame herself for the tragedy. She believes that she broke her promise to her brother—to take care of him always—and vows never to break another promise to a child.
Annie was virtually blind as a child, and she later attended the Perkins Institute for the Blind. There she underwent an operation to regain her sight. Her teacher at Perkins, Anagnos, taught her a great deal, and inspired her to become a teacher, too. Although Annie has regained her sight, she often wears smoked glasses in order to protect her sensitive eyes from bright lights.
Annie arrives at the Keller household, where she meets Helen. To the Kellers’ surprise, Annie uses a more hands-on, aggressive approach than any of her predecessors, getting down on the floor to interact with Helen and at times punishing Helen harshly for misbehaving. Most importantly, Annie begins by teaching Helen the sign language for things like “dog,” “doll,” and “cake.” It’s important for Helen to memorize these signs, Annie explains to a skeptical James, so that in the future she can understand that these signs symbolize real things. Helen is initially reluctant to cooperate with Annie. She hits Annie and, at one point, locks Annie in her room and hides the key. Annie is frustrated, but refuses to leave.
As the days go on, Annie succeeds in getting Helen to cooperate with her, but she realizes that she’ll never get Helen to learn sign language unless she can exercise total control over her pupil, independent of Arthur and Kate. Arthur is skeptical of Annie’s teaching methods, and at one point contemplates sending her back to Massachusetts. With Kate’s encouragement, however, he agrees to let Annie stay. Annie will have two weeks, alone in the garden house outside the Keller home, to teach Helen. Meanwhile, James continues to quarrel with his father. At one point, he accuses Arthur of “forgetting everything” after he married Kate.
Alone in the garden house, Annie makes gradual progress in teaching Helen how to be polite and orderly. She continues to teach Helen sign language, but recognizes that Helen still doesn’t understand what signs are. Meanwhile, Kate encourages James to “stand up to the world” rather than live in constant frustration with his father. Kate and James agree to be friends.
Two weeks go by, and Arthur and Kate prepare to move Helen back to their house. Back in the house, Helen immediately begins to regress, misbehaving when she realizes that nobody is going to punish her anymore. At supper, Helen spills a pitcher of water, and Annie angrily drags Helen out of the room. Arthur is about to prevent Annie from doing so, but James stands up to his father, asking him, “Has it never occurred to you that on one occasion you might be consummately wrong?”
Outside, Annie guides Helen’s hand over water from a water pump, and then makes the sign for water on Helen’s hand. To Annie’s amazement, Helen understands what Annie is doing, and even says, “wah wah.” Overjoyed, Annie cries, “She knows!” and shows Helen the signs for various other things, including “mother” and “teacher.” When Helen communicates with Kate for the first time, Helen spells out the word “teacher,” and Kate is both overjoyed and saddened. The play ends with Annie embracing Helen and telling her, in sign language, “I love Helen forever and ever.”