The last Metroid is in captivity the galaxy is at peace... Except for that Ridley is back to life *again* and he has stolen the last Metroid. And because all the other bounty hunters are, um, somewhere, you are the only one who can rescue it. All levels are beatable, except the first level sometimes and the second. If a level glitches out just press the flag and retry. (there is an ending cutscene) Green doors let you progress red doors kill you and purple doors warp you to strange places.
Thanks to -risen-