Ex-stalker? You can't have retired already, surely? D=
Well, that's up to you, haha. read all my fics But yes, it should be decently explained, I feel. In general though Pokémon Colosseum is not so unlike the main games as it is, unlike many other spinoffs. For one, battles work the same way (at least, with 3rd gen mechanics, so like RSE =p). The notable differences are that each battle was a double battle, so two Pokémon out at any one time. (In the game, anyway. ;p) So that should be already familiar to you.
The other major point is that the game had something called Snagging. Instead of catching wild Pokémon (hard because the game's setting is mostly desert/a ruined region!), in Colosseum you steal Pokémon from trainers via special Poké balls! You do this heroically though because these Pokémon you stole are 'Shadow Pokémon', Pokémon turned bad basically. Then you make them good again with the power of friendship (they sure love fighting for you yay battles).
Naturally that mechanic was heavily tied into the game's plot. And that should be also not so out-of-the-blue as the fic goes too. I hope, anyway. And that concludes my ramble.
How goes in rl, btw? Any luck in finding something else? :< And what about writing too, of any sort?