Aaron Kwok (郭富城) plays Ling Guang, a Hong Kong police chief inspector who is about to be promoted to superintendent, who lives with his wife Hazel (played by Taiwanese actress Chang Chun-ning/張鈞甯) and 5-year-old son Zai Zai (譚真一) in a beautiful house on Jagged Island.

The police are investigating a series of murders where the killer brutally “blood-drains” the victims, meaning holes are drilled into the victim's torso, and he or she is then left to bleed to death. Two seemingly unrelated victims have been found, and the third is police officer Tai, who drops out of thin air in a bloody mess from an apartment building entrance, right at the beginning of the film.

Oddly enough, Ling is just around the crime scene; he is found unconscious on the 7th floor of the building, with an injury to his head. In the hospital, he finds out he has short-term memory loss, and can't remember why he was at the scene with Tai, nor many of the other details of the case he has been investigating.

He tries to collect bits of information to piece together his memory and to find clues about the murderer. In the process, he is alarmed to find that all the evidence seems to point to him! Deep inside, however, he is convinced that he is innocent, and he becomes obsessed with proving it.

Only his colleague Ghost (Cheung Siu Fai/張兆輝) and Hazel believe Aaron is innocent. The rest of the Serious Crime Division is already investigating him. In his obsession to prove his innocence and protect his family from danger, Aaron's character changes from 'good cop' mode to 'fanatical madman' mode. He shows a range of emotions that make it clear why he is a two-time Best Actor award recipient.

We also see great performances from the supporting roles of Ghost and Zai Zai. One of the strangest things about this film, however, is that while everyone else speaks in Cantonese, Hazel delivers her lines Mandarin. The first of many occasions when the whole cinema burst out laughing (something that seems strange during a 'horror' lick) is when Hazel answers Ling's question in Mandarin. It sounds so weird, and they continue to do this throughout, even in the intimate moments.

Apart from that, the plot keeps audiences on the edge of their seats until the end.

All this time, Ling's short-term memory continues to plague him and he is losing the last trustworthy evidence – himself. Ling is eventually pushed to the edge of madness: Is he really the murderer, or not?