Pokemon is going through a Gale of Darkness, XD.
Basically, the plot revolves around a "sinister" and "evil" corporation that creates shadow Pokemon; and Shadow Pokemon are a bunch of **** Not that they treat you disrespectfully, but they just look like a bunch of **** Now, I'm a very judgmental person in video games (not proud of that) and I feel bad because those Shadow Pokemon are people too...XD (See what I did there? ... again?)
Anyway, I'd recommend Pokemon XD: Gale of Darkness to the people who made it through Pokemon: Colosseum all the way (100%) before trying Gale of Darkness. That way it will train you for the upcoming **** that are the Shadow Pokemon.