The premise of
Final Fantasy: Crystal Chronicles is that a deadly fog called Miasma has covered the world. However, there are special crystals of varying sizes that can keep the fog at bay. Once a year, the crystals need to be replenished with a special type of water called Myrrh. Myrrh can only be found at special trees called, surprise surprise, Myrrh Trees. A tree only gives a very small amount at a time however, and a single tree can only give Myrrh once every few years. Towns send out caravans to collect the Myrrh. They can't walk through the Miasma, of course, so the chalice they use to hold the Myrrh also has a tiny crystal on it that clears a small radius of the Miasma away. This is the basis for the game: you go out and collect Myrrh.
Not surprisingly, most of the Myrrh Trees are located in areas that are filled with monsters. So most of the game consists of you, and maybe a couple of your friends, fighting your way through monster-infested locales to get to the tree. Battle is a pretty simple thing in Crystal Chronicles and mainly consists of people pressing the A button to swing a weapon, or holding the A button down to cast a spell or execute a focus attack. You also have the Miasma to worry about. So someone has to carry around the chalice to prevent everyone from being hurt by the poisonous fog.
One of the most publicized aspects of Crystal Chronicles is its unique multi-player component. In order to prevent each player from disrupting the action of the game by going into menus, each player is required to use a GameBoy Advance to play the game using Nintendo's much touted 'connectivity' technology. Players navigate their menus on the GBA screen so they don't disrupt the game. Let's be honest. If you had four people playing together, you'd have to stop fighting every five minutes or so because someone would have to change up a spell or whatever.
You do NOT need to use a Game Boy Advance if you are playing the game solo. Battles are certainly easier with some friends, but you can access almost every area of the game if you are playing by yourself. It just requires some fast finger work, or a little ingenious use of your moogle friend who helps you out during single player mode. In fact, I can only recall one situation where I couldn't get somewhere by myself, and it goes without saying that it wasn't really a requirement to progress.
Spells are used after a player acquires the appropriate magicite in an area. You lose the magicite when you finish an area however, so you must require it every time you enter a dungeon. Players can combine the six basic types of magic to form more powerful spells like Holy, Holyra, Haste, and Gravity. You can also combine focus attacks and magic to do things like Flame Strike, etc...
As far as the progression of the story, it's mainly relegated to minor and somewhat random cutscenes you come upon as you travel across the world map. These can be rather redundant however, and I frequently ran across the exact same scene more than once. There's also very little actual 'progression' of the story. Towns are really there for some minor bits of back story, and mainly just for buying/selling items and equipment creation. Most of the plot advancement occurs at the very end of the final dungeon. While it's not quite as hollow as the story in Final Fantasy Tactics Advanced, the story in Crystal Chronicles is very light and not terribly deep or involving.