Bong ju (1982)

It has to do with a young clan member, Zhong Yuan, who is promoted to chief of the Dragon Gang after his adoptive father dies, only to be expelled when a letter arrives revealing him to be of Mongol birth, an awkward bit of news at a time when the Dragon Gang is actively fighting Mongol occupiers. In fact, Zhong soon learns that the Mongol general leading the anti-rebel campaign is actually his real father. Still, he persists in trying to get back into the good graces of the clan, eventually helping to ferret out a traitor in their midst.