Present at the assassination attempt of Taiwan's president, righteous cops Yuchang (Joseph Chang) and Sun (Simon Yam) spearhead an urgent investigation into the attempted killing. However, the likely assassin, mob hitman Jinshui (Gordon Lam), is curiously removed from suspicion when their superior officers instead pick inept loser Ertong (Liu Kai Chi) as the supposed killer. After questioning his superiors, Yuchang is removed from the case, but he and Sun decide to continue their investigation without official sanction. Yuchang travels to Taiwan to Hong Kong and back to uncover the true mastermind behind the plot, along the way uncovering an insidious, tangled web of corruption that stretches all the way to the highest office in the country.

Kong's perennial muse Stephy Tang stars as a reticent flower shop girl who rejects the overtures of a budding singer (labelmate popstar Justin Lo, Love at First Note), because of a reason she doesn't want to reveal. Pop diva Kay Tse makes her movie debut opposite stage actor Joey Leung as a modern couple whose relationship has run its course and withered to the test of time and money, but the memories of yesterday's love return during a dinner date. Funnyman Sammy Leung juggles multiple girls as a two-timing boyfriend who maintains his cheating charade by pretending to have a twin brother, but chaos ensues when his dates run into each other.

DJs Donald and Jim of gag pop group I Love You Boyz also keep things light as loveless goofs banking their Valentine's Day on a date with someone going by the name Goldfish, who turns out to be Miki Yeung (Fatal Contact) in skimpy clothing! Terry Hu (Love is Not All Around) and Chelsea Tong (Love is Elsewhere) play chaste budding lovers trying to keep their office romance a secret, while a heinous boss (Toby Leung, The Haunted School) breathes down their necks looking for her boyfriend's (Terence Siufay, Forgive and Forget) lover.

In the film, Wong Jing directs a star-studded cast that includes Tony Leung Ka Fai, Anthony Wong, Eason Chan, Alex Fong, Bowie Lam, Kate Tsui, Liu Yang, Natalie Meng, recently crowned Hong Kong Film Awards Best Actress Paw Hee Ching, and Wong Jing himself. The powerful Chinese Chief Inspector Lak (Tony Leung) and his cohorts Lun (Anthony Wong), Gau (Eason Chan), and Chu (Wong Jing) have practically turned half of Hong Kong into their corruption empire. This prompts the Hong Kong Governor to set up an anti-corruption arm to fight them. Idealistic university graduate Han (Alex Fong) joins the newly formed ICAC headed by Yim (Bowie Lam), and their war against corruption will prove decisive for the fate of Hong Kong.