The One - The whole movie is based on the idea of multiple dimensions and was basically set up so Jet Li could fight Jet Li. Anyway Jet Li 1 (I forgot the characters name) has gone to all the different dimensions killing all the other Jet Lis (I thought there were infinite dimensions) and the more he kills the more powerfull he gets. The only Jet Li left is Jet Li 2 who lives in our dimension so Jet Li 1 tries to kill him so he can be invincible.
EDIT: Heres the plot summary from wikipedia, it makes even less sense than mine
A superhuman criminal named Yu-Law (Jet Li), once a member of the organization policing interdimensional travel (via detecting wormhole openings, which can be predicted like the weather), seeks to hunt down and kill variations of himself in alternate universes to absorb a common life force (which possibly has to do something with the string theory as it was mentioned in the movie by Evan Funsch) that connects them all. By killing his other selves, (becoming the last version of himself) and absorbing their power, he thinks he will become a god.
The only one who can stop him is the last of his alternate selves (MVC: MultiVerse Counterpart), Gabriel Law, who works in the Los Angeles County Sheriff's Department in a near-image of our own universe (George W. Bush remained President, but had some policy differences such as a "Universal Health Care Plan") and, before the events of the movie, was unaware of the multiverse. In the attempt to kill Law, Yu-Law starts a fight where he is hiding in an airshaft. After shooting where they believed he was hiding, the Sheriff's Department attempts to move in, but Yu-Law busts through the metal and takes out the entire team, single handedly. He then has to flee on foot when more troops move in. He is seen "Doing fifty!" running down the streets from the police. In the end, Law was the only one who could keep up with him, not to mention barely jumping the tall fence. Law then sees Yu-Law who is identical to him in appearance, and Yu-Law says "The good news is, you're not crazy." Law is assisted by a member of the Multiverse Authority that Yu-Law once worked for, with the concern that if only one version of Yu-Law is left there will be catastrophic results for the entire multiverse.