Jerusha "Judy" Abbott was brought up at the John Grier Home, an old-fashioned orphanage. At the age of 17, Judy is informed by the asylum's dour matron that one of the trustees has offered to pay her way through college. Judy catches a glimpse of the shadow of her benefactor from the back, and knows he is a tall long-legged man. Because of this, she jokingly calls him Daddy-Long-Legs. She has an obligation to write him a monthly letter, but she will never know his identity; she must address the letters to Mr. John Smith, and he never will reply. The letters chronicles Judy's educational, personal, and social growth as she attends a "girl's college" on the East Coast.