Just like Colosseum, this game is different from the rest of the series, but don't let that stop you from enjoying it.
Pokémon XD: Gale of Darkness takes place 5 years after the events of Pokémon Colosseum, where young Michael (his default name), his sister Jovi, and his mother live and work at a laboratory in the mostly desert region of Orre. After a handful of events happen involving the evil team known as "Cipher", which is revealed to still be together after the events of Colosseum, Michael journeys to stop them, as well as capture and purify the Shadow Pokémon, Pokémon that have became evil when the door to their hearts were closed, which Cipher creates.
If you don't like double battles in Pokémon, I warn you that almost every battle in this game is a double battle. I didn't like this at first, but I soon came to enjoy it.
Something I really admired is the fact that the fight with the admins of Cipher remind me of Gym Leader battles, I don't know how other people think of it.
The story, gameplay, and especially music are phenomenal. I actually think that XD has the best music in the series.
The battles are a little slow, and the game is overall different from the rest of the series, but don't let that make you think that this game isn't good, because you might find that Pokémon XD: Gale of Darkness is a really great game, and a great entry in the Pokémon main series.