Whilst searching for his mother, Gouzazhong a.k.a. 'Mongrel' (Philip Kwok) finds a metal tablet amidst a fight between several martial art clans. A passing couple Shi Qing (Tang Ching) and his wife Min Rou (Lau Wai-ling) immediately recognize the item but before they can explain its significance, it is taken back by its original owner, Xie Yanke (Wong Lik), a powerful and eccentric martial artist. Xie confronts the fighting clan members and proclaims he will punish them in a ritualistic massacre he conducts once every three years. Xie, having promised whomever can find the tablet any favor they wish, pushes Gouzazhong to name his, offering him gold and even to teach him kung-fu, but the young man is uninterested and rejects all his offers. Enraged, Xie forces him to study kung-fu but purposely teaches him the wrong techniques. Gouzazhong falls unconscious during his training and is left to die, but is discovered by Bei Haishi (Sun Chien), second-in-command of the Changle Gang, and taken back to their headquarters to recuperate. That night, a young woman named Ding Dang (Candy Wen) slips into his room and despite having never met her, she begins acting intimately with him, calling him Shi Zhongyu and leaves before promising to visit again. He is then visited by Bei Haishi who acknowledges him as their leader and again, calls him Shi Zhongyu despite Gouzazhong's protests to the contrary. Gouzazhong is later attacked by one of the Changle members seeking revenge on Shi for accosting his wife but the strike inadvertently hits an acupoint granting him immense strength.
Ding Dang later returns and despite Gouzazhong's protests that she has the wrong man, claims the real Shi has a bite mark on his shoulder and challenges him to show his; sure enough, he indeed has the marks, and is forced to marry Ding Dang by her grandfather who is of the belief he is feigning amnesia to abandon her. Bei arrives to take Gouzazhong back as several martial artists from the Snowy Mountain School have arrived at the gang's headquarters. One member, Bai Wanjian (Chan Shen) demands retribution against Shi Zhongyu who, despite being sent to the school to learn martial arts, assaulted his daughter causing her to attempt suicide, and had killed another disciple before fleeing. When Gouzazhong insists they have mistaken him for Shi, Bai proves he has the right man by asking him to show his thigh, revealing he once left eight marks there with his sword when Shi once showed him great disrespect. Again, the marks are present, leaving Gouzazhong baffled. Shi Qing and Min Rou arrive at the Changle Gang base where it is revealed they are Shi Zhongyu's biological parents and that twenty years ago, one of their sons and Shi Zhongyu's twin brother, Shi Potian, was abducted and murdered by a woman by the name of Mei Fanggu (Teresa Ha). Bei Haishi later reveals to his gang members he had known all along that Gouzazhong was not their real leader Shi Zhongyu but kept him around nonetheless, even marking his shoulder and knee when he was still unconscious due to the fact Xie Yanke plans on killing each of the Changle Gang members unless their leader agrees to take their place. Bei thus plans on manipulating the young and naïve Gouzazhong into being their scapegoat.
Gouzazhong and Ding Dang eventually return to the Changle Gang with the intent to step down as their leader. The meeting is interrupted when Xie appears, bringing with him the real Shi Zhongyu (also played by Kwok) who looks identical to Gouzazhong. He not only refuses to help the Changle Gang but remains unrepentant, refusing to return to the Snowy Mountain School to face punishment. As Xie prepares to kill all the Changle Gang members, Gouzazhong volunteers to fight him and die in their place should he lose. He manages to disarm Xie but spares him, and Xie leaves while Shi Qing, along with the Snowy Mountain School, resolve their mistaking of Gouzazhong's identity and leave with Shi Zhongyu. Bei and the other gang members, moved by his selflessness, pronounce Gouzazhong their true leader and offer to help him find his mother. Ding Dang returns once again that night, this time tricking Gouzazhong into switching places with Shi Zhongyu, who faces death for his crimes. The ploy however, is discovered by Xie but Gouzazhong spares Shi Zhongyu from being killed by him, instead asking that Xie take him under his wing and rehabilitate him. Shi Qing requests that Gouzazhong take him to see his mother, revealed to be Mei Fanggu who had also been Shi Qing's scorned lover, thus the reason she kidnapped his son when he was born. Mei had in fact secretly raised Shi Potian, renaming him 'Mongrel' to spite his parents and hoped to have him kill Min Rou when he reached adulthood; in his place was a dead infant she found on the street that she returned to Shi Qing and Min Rou. She takes Shi Qing hostage and demands that Gouzazhong kill Min, although he refuses and, with help from the Changle Gang, takes on her bodyguards, White Tiger (Chiang Sheng), Azure Dragon (Chu Ko), and Vermilion Bird (Ricky Cheng). During the fight, Bei successfully slips away to free Shi Qing and holds Mei at knifepoint while Gouzazhong kills the three guards. Shi Qing ultimately decides to spare her and departs, reunited with his long-lost son.
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