The Miracle Worker is based on the involvement of Anne Sullivan in the life of a young Helen Keller, left blind and deaf after an illness when she was a baby. In formulating the story, William Gibson, used Anne's actual letters and the autobiography, The Story of My Life, by Helen Keller herself. 

Annie Sullivan's first job is to educate Helen. She is only partially-sighted herself, having had an operation to restore her sight and aged only twenty. She has a different background from the Keller family and must learn to adjust. Helen's father is mystified as to how anyone could possibly expect this "child herself" to teach his obstinate daughter. He also finds Annie's behavior to be disrespectful.  Helen's mother, Kate, is so desperate that she persuades her husband to let Annie try anything in improving Helen's chances of communicating. 

It is Annie's persistence and enduring patience that never change but her relationship with Helen, after the "miracle," changes dramatically. Previously, Helen fought with Annie over everything and Annie takes whatever steps necessary. Despite being a strong-willed, opinionated and confident person,  Annie has her own issues to deal with as she and her brother Jimmy were placed in an institution as children. Annie promises to take care of Jimmy but he dies and she is haunted by this memory. When left to care for Helen, Annie questions herself, the voices she hears serving as her conscience. Only at the end, when the "miracle" occurs, can she move on from this and dedicate herself to helping Helen, loving her "forever."