Adapted from the novel Babylon Babies by Maurice G. Dantec, Babylon A.D. stars Vin Diesel as Toorop, a mercinary hired to escort mysterious Aurora (Melanie Thierry) from Russia into New York City. Accompanying them is Sister Rebecca, played by the ever-wonderful Michelle Yeoh, a peaceful nun of the Noelite religion.
After that, I can't really tell you what the movie is about because 20th Century Fox cut the hell out of it. It's a jumble of action scenes and Vin Diesel grunts after about halfway through. In France, the film is at least ten minutes longer, though I've heard rumours of a two hour and ten minute version. Director and adapter Mathieu Kassovitz has stated publicly that Fox wrested control of the film from him and cut out the moral implications of the violence and you feel it, especially at the end. It's a mess. Absolutely hacked to pieces.
I've never read the book and I understand that Kassovitz took his own liberties with the film, including adding more of Sister Rebecca in order to have Michelle Yeoh in the film longer. To be honest, that was a brilliant move as she's the heart of the picture, especially in the Fox cut. I wish he'd gone with his original casting choice for Toorop, the brilliant Vincent Cassel, but I think that I'll reserve my judgments on Diesel's performance (and Melanie Thierry's) for when I see the cut that Kassovitz intended.
Fox has a long history of interferring with their directors. Alien3 is possibly the best example of their butting in. The workprint version of Alien3, included with the Quadrilogy, is much better than the theatrical. I only ever watch the workprint version, which wasn't ever completed because David Fincher was removed before he could oversee the final touches needed.
If you're only looking for action, cool set design, great cinematography and okay music, then go see this flick. If you were hoping for the story that comes with the name Babylon Babies, then don't bother until Mathieu Kassovitz's version is released.