From what I can tell this game seems to take place right before X4. Iris is still alive. X and Zero have stumbled upon an island that is flowing with mavericks. They find out that the Island is being run by two mavericks by the name of Berkana and Gareth. They are called Soul Erasers. They are stealing reploid "DNA souls" and using them to make themselves more powerful. Needless to say, X and Zero must stop them with Iris' help.      I decided to put an overview for this game because it is very strangely set up. When you first start the game you'll be given the option to play the Zero game or X game first(I started with X). When you complete one, you will them start the other. Each character has a different set of 4 mavericks to destroy. X has to fight Neon Tiger, Launch Octopus, Volt Catfish, and Flame Mammoth. While Zero has to fight Wire Sponge, Blast Hornet, Overdrive Ostrich, and Tunnel Rino. After you comlete one game, Iris tells you to save your game, and then you will be set up to play the other character's game with the weapons that you would have picked up had you been playing with the previous character. After you've finished both X and Zero's game, you can go onto Xtreme Mode. You'll fight all 8 Mavericks and you'll be able to switch between X and Zero whenever you want. The only bad thing is that whatever items you pick up go right to that character and not to the other. So if you play through the whole Xtreme mode with just X, Zero will not be able to use any weapons or hearts that X picked up. The only item that is shared between X AND Zero is the sub tanks(which there are only 2 of in this game). Like Berkana and Gareth you can also use the DNA souls to power up. These power up parts will increase your abilities, such as how much damage you deliver and recieve. The DNA souls are little capsules that the enemy drops. The capsules come in clusters of one, two, or three pieces, and can restore life or weapon energy as well.
Overview
     The enemies in this game are REALLY EASY!! I found a few enemy weaknesses but I rarely used them. When you start to pick up the +1 and +2 power up parts for X and Zero, the game becomes rediculously easy. Below is the order I fought the enemies in. You may notice that Neon Tiger's weapon works on both Volt Catfish AND Launch Octopus. I'm not sure why, but if it works, use it!
The order of bosses I fought them in (I also fight them in this order on Xtreme Mode):
X's Game
   1. Neon Tiger
   2. Launch Octopus
   3. Volt Catfish
   4. Flame Mammoth
Zero's Game
   5. Wire Sponge
   6. Tunnel Rino
   7. Blast Hornet
   8. Overdrive Ostrich
The order of the bosses by weakness is:
   1. Neon Tiger
   2. Volt Catfish
   3. Launch Octopus
   4. Flame Mammoth
   5. Wire Sponge
   6. Overdrive Ostrich
   7. Blast Hornet
   8. Tunnel Rino