Kingdom Hearts is, and has always been, something of an oddity. Part of me wishes that I'd been there to listen in on whatever meetings spawned this bizarre fusion of Disney and Final Fantasy (and that I, uh, spoke Japanese), because it's such an unusual and far-fetched concept. Crack-conceived though it might have been, that isn't to say it didn't end up working out well: though not without some glaring flaws (looking at you, camera system), the first Kingdom Hearts was a charming and successful game, and people loved it. In order to bridge the gap between Kingdom Hearts and its inevitable sequel, Square released Kingdom Hearts: Chain of Memories for the Game Boy Advance - now properly joining its two brethren on the PlayStation 2 with the remake, Kingdom Hearts: Re: Chain of Memories.
As remakes go, it's remarkably faithful to the source. If you've played the GBA game, imagine it in full 3D a la KH1 and KH2, and that's pretty much what you'll get, for better or for worse. The cutscenes are now ... well, proper cutscenes, complete with voice acting, and more than a few cast members reprise their roles from the other two games. If you liked CoM on the GBA, there's a pretty good chance you'll like this remake.
Chain of Memories starts quite literally exactly where the first Kingdom Hearts ended: that game's ending makes up a good chunk of the sequel's introduction. Sora, the series' plucky young hero, and his adventuring companions Donald Duck and Goofy are off searching for Sora's friend Riku and King Mickey Mouse, who vanished at the end of the first game. CoM assumes prior knowledge of the series and story, and despite a half-hearted attempt to catch everybody else up via a short montage of flashback clips, the plot doesn't really work without that knowledge. If you never played KH1, you won't really know who the characters are, or how they relate to each other, or how the Final Fantasy cast fits into the mix ... and you probably won't give a damn.
Sora & Co. make their way to a mysterious fortress called Castle Oblivion, and as they venture inside progressively lose more and more of their memories (conveniently stripping them of all their abilities and combat prowess that they learned by the end of the first game). In Castle Oblivion, explains one member of the cloaked-and-hooded Organization, everything is determined by cards, which is the main thing that sets Chain of Memories apart from the other two Kingdom Hearts titles - the card-based gameplay.
When they say that everything is determined by cards, they really do mean everything. Sora's fading memories of his adventures are transformed into cards representing the various Disney worlds he visited from Halloween Town to Agrabah, and between each floor in Castle Oblivion you get to choose which world you want to go to next - in whatever order you feel like playing them in. Each world consists of a series of connected rooms, and every room has a certain requirement to unlock it that can only be filled by - surprise, surprise - using cards.
The requirements vary from room to room (use a card with a value higher than 3 or lower than 5, or use a blue card - to name a few) and the specific card that you use determines what the room will contain. Some cards create stronger, more aggressive enemies that actively hunt you down, some cards create rooms full of merchants who buy and sell cards, some rooms boost the power of your magic attacks, and that's just scratching the surface. Via the card system, you effectively have the power to customize your journey through Castle Oblivion however you see fit (to a point, anyway), which is a pretty nifty concept and makes the game feel like it's less of a rehash of the first Kingdom Hearts than it actually is.