"The story began one winter.

It snowed heavily that day. William was still a cripple with a crutch, and the white bearded old master fell at the door of his house. Out of kindness, he called his father to save him. Unexpectedly, the old man was a visiting doctor. He had made a vow that he would never practice medicine for the rest of his life. In order to repay his kindness, he broke his promise. After he cured Zhang, his father asked him to be a doctor, but the doctor refused. However, he left a medical book. Before he left, he told him that there was a single line...