Blood of the Dragon Peril.

The Master of Manchuria's most renowned school of kung-fu is slaughtered when he resists the invading Japanese. His eldest son is beaten so badly he becomes mentally retarded. The second son, Lui, surrenders and joins the army - much to the derision of the villagers. The last son disappears. When a masked man begins appearing and making trouble at the Japanese army bases, Lui is ordered to find and kill him. He almost succeeds, but at the last moment discovers the masked man's secret...