Any Pokemon fan will really enjoy this film, even if (like me!) they're purchasing it a bit late.
The story basically follows Ash, Brock and Dawn (no surprise there) when they, along with an entire town and a host of quirky characters such as the arrogant Baron Alberto, get trapped in the middle of a fight between the Pokemon deities Dialga and Palkia. With the appearance of the mysterious Pokemon Darkrai, the townsfolk begin to find odd things beginning to happen: seemingly solid rock becomes inexplicably twisted, the people find themselves unable to leave the city, and the realm of dreams and reality begin to merge together. A showdown of epic proportions ensues, and the outcome will determine whether the town will survive or be wiped from reality entirely.
As far as Pokemon films go, this one is definitely up there when it comes to the quality of animation, and the soundtrack is brilliantly dramatic. The fight scenes are to die for and will definitely have you gawking. One of the best Pokemon films I've seen, and if you can get into it it's incredibly addictive to watch. :)