The Miracle Worker portrays the turning point in the childhood of the famous author and activist Helen Keller. As a baby, Helen suffered an illness that left her deaf, blind, and mute. The play follows the arrival of her teacher, Annie Sullivan, who changes Helen’s and her family’s lives through tenacity, patience, and hope.

Set in Tuscumbia, Alabama, in the 1880s, the play portrays a family torn apart by sickness, disability, and resentment. Helen was born able to see, speak, and hear, but an illness erased her chances of a normal life. Her parents, Captain Arthur and Kate Keller, feel the best they can do is appease Helen’s frustrations by offering her items to keep her complacent. Without the ability to communicate, however, Helen runs amok, destroying the home and the family’s peace.

When Helen acts up, Kate, Arthur, and their servant, Viney, all give her food to soothe her frustration, which ultimately reinforces Helen’s bad behaviors. The family, pitying Helen, has never once disciplined her. James, Arthur’s son from a previous marriage, is the only person who feels rewarding Helen’s bad behavior is a detriment to her life, but he is too afraid of his father to make his point heard.

There seems to be no hope for Helen, until the family is sent a young teacher named Annie Sullivan, a twenty-year-old Irish woman from Boston. Annie faced many hardships as a child. She grew up in an insane asylum and lost her brother at a young age. She also contracted trachoma, which caused her to go blind until a series of surgeries helped her regain her vision. She spent her formative years at the Perkins Institution for the Blind, and upon graduation, she was sent to Alabama to help the Keller family.

When Annie arrives at the Keller home, the family is not comfortable with Annie’s confidence and “Yankee” attitude, believing she is far too young to offer assistance, but Annie believes she can help. She captures Helen’s attention when they first meet by signing letters into Helen’s hand to begin teaching her the alphabet.

However, trouble occurs in the first few hours at the Keller home when Helen knocks out one of Annie’s teeth and locks her in her room. Annie realizes that not only does she have to teach Helen to communicate, but she also has to teach Helen basic manners. Her main goal becomes teaching Helen and her family the word “no.” But Annie also witnesses an important moment: without Helen’s awareness, she watches Helen take the key to her room and throw it down the well. Annie realizes Helen has a bright mind with much to offer.

At their first meal, things get worse for Annie and the Keller family. Helen begins grabbing food from people's plates and throws a tantrum when Annie attempts to correct this behavior. She grabs Helen’s wrists and physically reinforces the concept of “no.” She makes it very clear to Helen and the family that this type of behavior won’t be tolerated. Arthur disapproves of Annie’s attitude and her tactics as Annie forces Helen to pick up her mess. The family is bewildered when they see Annie work, but Annie knows Helen needs discipline before she can learn to communicate.

After a few small breakthroughs, Annie asks the Kellers if she can separate herself and Helen from the home by staying with Helen in the Kellers' garden house. Arthur is vehemently opposed to this idea but eventually gives in and tells Annie she has two weeks to do things her way. The Kellers will not be allowed to communicate with their daughter in any way, but they can look through the windows from time to time to check in.

Through the use of tough love and discipline, Annie is able to show Helen how to behave in a civilized manner, and she teaches Helen several words through signing. She never lets Helen do something without imparting a consequence or lesson. By the end of the two weeks, Helen has learned how to eat properly, fold her napkin, and use silverware. She also starts remembering the words Annie has taught her and can stitch on her own.

The Kellers retrieve their daughter and move the girls back into the main house, but Annie isn’t satisfied. She feels she needs more time with Helen to reinforce her lessons and the idea that the words she is spelling have meaning beyond the letters. She concedes to the original plan but begs the family to follow her rules by disciplining Helen. Arthur is deeply grateful for the progress Annie has made, and he promises to help Annie keep Helen in line.

At their homecoming meal, Helen throws a tantrum, and the family immediately gives in to her behavior. Annie tries to explain that Helen is testing them, but the group refuses to listen to Annie. Annie is furious and fights to uphold the standard she has set as Helen repeatedly drops her napkin on the floor.

Annie gives Helen a chance to correct her behavior, but when she fails, Annie grabs Helen’s wrists and corrects the behavior herself as if it’s the first meal all over again. Arthur and Kate are unhappy with Annie’s reaction and feel she is ruining Helen’s celebratory meal.

After Helen spills a pitcher of water, Annie takes her outside to refill it. When Arthur gets up to follow the girls in protest, James finally stands up to his father and prevents him from leaving the room. Arthur concedes to his wishes, and the family remains seated.

Outside, at the well, a miracle occurs. While Helen pumps the water, Annie spells “water” into Helen’s hand. Helen suddenly stops and drops the pitcher. They both freeze until Helen says, “wah wah.” She grabs Annie’s hand and starts spelling the word “water.” Helen starts touching everything around her and gestures for Annie to spell the word for each object. Helen realizes the letters and words have meaning and that each thing she touches has a corresponding word.

Annie yells for Kate, and the family rushes outside to find that their daughter is able to communicate with the world and with them. Helen spells “Mother” and “Papa” into the hands of her parents, then rushes over to Annie to understand her role. Annie spells “teacher,” and at that moment, they are bonded for life.