Borrowing from the Many Worlds interpretation of Everett's Relative-State Formulation of Quantum Mechanics, The One showcases a scenario of what happens when events in these different universes become intertwined.
The setting for this plot is a "multiverse" consisting of many parallel universes (using the Everett interpretation, there are an infinite number). The first Universes that discovered this "fact" went about creating a police force to ensure no abuse occurs by travelling from Universe to another through "Quantum wormholes". Yulaw (Jet Li) is a former officer in this force who discovers that his strength increases when he kills a version of himself from another Universe. He then goes about killing the 123 copies of himself that are known to exist and their strengths flow to him and his one remaining counterpart, Gabe (Li again), in the 124th Universe (arguably there are more than 125 Universes, but a Yulaw-like personality does not exist in them).
Gabe, a regular cop in a Universe similar to ours, is unable to explain his sudden increase in strength. That is until Yulaw comes after him. Aided by two of the Multiverse police, Roedecker (Delroy Lindo) and Funsch (Jason Statham), Gabe must stop Yulaw before Yulaw becomes "the one" standing, which would allow him to gain an enormous amount of power and rule the Multiverse as a despotic god.
The idea that as one person is killed in a given Universe, his strength flows to all his other counterparts in different Universes, is an interesting one. The corollary is that all the counterparts of a given individual follow some Zen-like equivalent of the First Law of Theromodynamics (that their strengths are conserved). That also is the flaw in the idea driving the film: why make a distinction that limits strengths to individuals? Why not atoms or molecules? If the strengths holding together the atoms that compromise an individual were distributed to all atoms in all Universes when that individual died, then assuming an approximately equal number of births and deaths, no one would notice anything. (Presumably also, in the natural order of things in this film's Multiverse, a counterpart would be born every time one dies to ensure there is a balance.)
When I saw the previews for this film, I thought that watching Jet Li fight with himself should be simply amazing. But it wasn't. The movie is interesting for the ideas it brings up, but there is very little of Jet Li's martial arts prowess on display here. The film is fast-paced and by the time the basic elements of the plot are set up, there's little time for fight sequences.
The soundtrack for the film is very good. The joke of Al Gore as U.S. President in one Universe was topped by George W. Bush proposing universal health care in another. The One is a great film to watch on the big screen, especially if you happen to like mostly brainless action.