The only clue Chiang has is that his brother is a Muay Thai boxer with a tattoo of an anchor on his arm. They both meet, unaware of their relationship, and become good friends. The two brother's relationship becomes clear when Lung takes off his robe for his match against killer. When Chiang sees the tattoo he tries to stop the match but fails and has to watch his brother get battered by Killer's deadly knees. Lung nearlly gets killed but manages to make a come back after performing a critical double front kick.
Lung wins the match and both brothers get to know each other better.
This film has a similar story to Chang Cheh's later work, Death Ring . David Chiang's character is quite mysterious in the sense that you're not sure as to how good his kung fu is. I think Chang Cheh was trying to show that Chinese martial arts is much better than thai kickboxing. I cannot disagree.