Sunny Li Fei [Aaron Kwok] is the orphaned child of a former knife-throwing master. After his father’s death, his uncle takes him in and lets him perform as a clown in a circus, since he did not inherit his father’s knife-throwing skills. Sunny longs to become a real knife thrower, though, and is constantly taunted by Collin Chou as Zhang Dachu and his cousins, fellow performers in his uncle’s troupe.
One day, Sunny overhears the troupe planning to explore a cave rumored to be filled with gold. Instead of harming Sunny, they use him as the point man for the expedition. They uncover crates packed with capsules and force Sunny to open them. Inside the first capsule lie plates of gold, and as they continue to break open the rest, the experimental gas is released, mutating the group into superhuman monsters. In the ensuing chaos, Sunny accidentally kills Zhang Dachu’s younger brother and manages to escape the troupe’s wrath, but he himself is exposed to the gas and collapses on a smuggler’s ship bound for Hong Kong. He is thrown overboard when the crew discovers him days later.
A few days later, Angel Chan [Shu Qi], a reporter, finds him in an obese state after he wakes from a coma and is brought to a safe house. While sleeping, Sunny drains his fat and excess water, and when he awakens, he is back to his normal form but now possesses superhuman strength and enhanced abilities. He tries to contact his uncle, only to learn that the uncle has been arrested by the Malaysian police for the incident, while incessant mosquitoes keep interrupting him.
Meanwhile, a wave of brazen robberies—armored cars, banks, and jewelry shops—unfolds, and several medical experts are abducted. A mutant couple disguised as detectives, Sun Hao [Wu Jing] and Cheng Xiuhua [Zhang Jingchu], decide to investigate the matter. Angel, demoted on her show for a younger rival, witnesses a hostage situation and sees Sunny disarm the hostage-taker with a simple stick, making him an instant hero. He inadvertently blows his cover to his mutated former troupe members, and the newfound fame also makes him a target, as they pursue him for his blood, convinced it contains the antidote to the virus.
The attack is repelled with the help of Sun Hao and Cheng Xiuhua, and one mutated troupe member is killed. This struggle also reveals Sunny’s new weakness to cold. Doctors determine that Sunny’s blood contains antibodies capable of neutralizing the poison, but it does not affect his mutant powers. Sun Hao and Cheng Xiuhua decide to become Sunny’s bodyguards, determined to protect him from Zhang and the surviving mutants.
During one commercial shoot, the troupe assaults again. Zhang Dachu subdues Sunny and drinks his blood, hoping to gain his power. To Zhang’s shock, the blood does not heal him; it accelerates the mutation instead. The Sun–Cheng Xiuhua duo escape with Sunny, but Xiuhua dies from her injuries. Sun agrees to train Sunny to help him battle Zhang and the remaining mutants.
Zhang, smitten with Angel, lures her to a hotel that turns out to be a police trap. He kills several officers before Sunny confronts him. Sunny defeats Zhang’s mutant cousin in a suicide-explosion moment, and he finally kills Zhang with his knife-throwing skill when Zhang injures Angel.
In the end, Angel and Sunny live together, and their family grows with a son.
