From Wiki:
The story was set in the Song Dynasty (960-1279) and at the beginning of the Nuzhen (Jurchen 女真) Invasion of northern China. The first part of the novel revolved around the friendship of two young village men, Yang Tiexin (楊鐵心) and Guo Xiaotian (郭嘯天) who became heroes in their own right as they fought the invading Jin soldiers. The bond between the two was so deep that they swore to each other their unborn children would become either sworn brothers or sisters (if both were of same gender) or be married (if they were of different genders).

The story focuses on the trials and tribulations of their sons following Guo Xiaotian's murder and Yang Tiexin's disappearance. Guo Jing (郭靖), Guo Xiaotian's son, grew up in Mongolia, under the protection of Genghis Khan. Yang Kang (楊康), on the other hand, grew up as a prince's son in the Jin Empire.