A team of military special forces are escorting a scientist and his family in the Middle East when they are ambushed by terrorists, intent on taking the scientist's work on a genetically engineered super-strain of smallpox and using it for their own private financial gain. One of the operatives receives a bullet in the head, but declines to stay in hospital to recuperate, instead heading off to Malaysia to look for his estranged father and the brother he never knew he had. As luck would have it, this course of action brings him back in contact and indeed conflict with the terrorists.
Hong Kong does not typically do hi-tech action thrillers very well, and terrorists seeking super-viruses is certainly a red flag when it comes to a Hong Kong movie. The smarter Hong Kong's scriptwriters try to make their characters sound, the dumber they usually come across, and this is no exception. Additionally, credibility-stretching coincidences are rarely a hallmark of a well-written script. One incredible coincidence which becomes the centre-piece of the story, exploring how the effects of random probability or the hand of fate can transform a person's life, can make for an interesting story and film. When your hi-tech action thriller introduces at least 3 unbelievable coincidences in an attempt to make your story about super-viruses seem more coherent, you should probably realise you've taken the wrong track.
In short, the script for The Viral Factor is a mess... ridiculously, eyeball rollingly so, really. The annoying thing is that it's all quite unnecessary... at its heart there is a story of two estranged brothers that have found themselves on opposite sides of the law, but who have to cooperate to protect their family and see if they can find forgiveness and redemption. The implausible coincidences don't really add anything to that scenario, and the whole terrorists with super-viruses aspect is basically not needed, too. They're plot devices that speak of a weak writer, one who feels that the simple motivations of family, love, guilt and redemption just aren't enough to engage the viewer... when in fact they're the only parts of the script that do.
Somehow, the glaring flaws in the story do not derail The Viral Factor nearly as much as you feel they ought to. The writing is awful, but pretty much everything else about the film is good. Dante Lam's direction is strong, the performances from Jay Chou and Nic Tse are good, visuals and production values are excellent and the action scenes are top calibre. Some individual scenes are well written and executed, and there is generally a lot to like about the film. Generally, the further the whole 'Viral Factor' aspect is pushed into the background, the more engaging and enjoyable the film is - and for much of the running time it is indeed relegated to the background.
If Lam had had the confidence to just excise all the hi-tech espionage type fluff and focus on the emotional core of the story, he could have made a great film here... though admittedly he'd have robbed himself of some excuses for staging some quite remarkable action scenes (though I'm sure he'd have found a way). As it is, I guess he can take some credit for managing to produce a pretty solid film despite the self-imposed handicap of a ridiculous script.