Annie Sullivan comes to the Keller home in order to work with Helen and give her mind and soul some "light" into what goes on in the world she cannot see. Annie hopes to be able to teach language to the seven-year-old deaf and blind child.

As Helen becomes increasingly difficult and efforts to find someone who can work with her have failed, Mrs. Keller and Aunt Ev try to persuade Captain Keller to contact a doctor in Baltimore, who has accomplished things with other blind children. Finally, with the suggestion of Alexander Graham Bell, the Kellers make contact with the Perkins Institute for the Blind in Boston, and the director sends twenty-year-old Annie, a legally blind student herself, to them as a governess. 
Annie herself is given to battles of the will. But, she has overcome the tragedy of poverty, losing her brother, and the disease trachoma which severely damaged her eyesight. She learned to read braille and sign language and became valedictorian of her class.
When she arrives at the Keller home, she tells Mrs. Keller, who is concerned that she is so young:

Mrs. Keller, don't lose heart just because i'm not on my last legs. I have three big advantages over Dr. Howe that money couldn't buy for you. One is his work behind me, I've read every word he wrote about it....Another is to be young; why, I've got energy to do anything. The third is, I've been blind.

After many struggles with a determined Annie and battles with her family, Helen finally breaks through her darkness. Annie's accomplishments earn her the description by Mark Twain as the "Miracle Worker."