The story takes place just after the Game Boy's “Metroid II: Return of Samus". Samus has exterminated the Metroids except one hatchling that seems friendly towards Samus. She takes it to a medical research facility. Shortly after Samus departs the facility, Ridley attacks, takes the Metroid and returns to the space pirate world, Zebes. Samus returns to Zebes to take on the space pirates and rescue the Metroid.
I criticised the previous game for having poor games design, and Nintendo have learnt their lesson here. The opening of the game gives you time to get used to the movement and platforming. After a short battle with Ridley, you get a section that introduces exploration by making you walk past many doors you cannot access until you find the appropriate power-up. After finding the morph ball and missiles, you then can backtrack and reach new areas. This is a great way of teaching the player the mechanics and structure of the game.
The first part of the game is clear where you have to go. At a certain point, it becomes more vague. I found I was often backtracking a fair distance to find a new item, then end up backtracking a similar route. This slows down the pacing after the midpoint of the game. I know there will be a more optimal path through the game, but without looking at walk-throughs, you will be going back and forth until you stumble upon that key item.
One big problem with the original game was that there were too many enemies and they rarely dropped health pick-ups which barely restored your health. In this game, the enemies are often easier to take down, they drop health more frequently, and these pick-ups restore more health.
Even though enemies drop frequent pick-ups, I did find myself grinding these at certain times. There are pipes in some rooms that spawn infinite enemies, so you end up standing there gunning them down for several minutes. There are a few rooms which can restore either health or missiles but these were scarce. Returning to your ship on the surface restores all health and missiles but this requires some severe backtracking. I thought a better option would be to restore health and missiles when finding the save station. Allowing you to teleport between stations would have been a huge convenience too.
You also have a map which marks where you have been. Initially, the map draws as you explore but there will be a device that populates the map for that area. The map isn't detailed as it only shows you rooms but not the colour of the door; so you can't see how to access it. (Some doors require certain power-ups to open). Some areas might not even have a door, and sometimes two rooms may be next to each other but aren't actually connected. Other rooms may be accessed by bombing the wall. These hidden paths can be annoying because they are still too frequent. Sometimes you had a suspicion there could be a path there, whereas others you may find just by dropping a screen-clearing Super Bomb. You can find a scanning device later in the game that allows you to spot these items but you have to actively use it.
You do feel very powerful by the end since you can carry a crazy amount of missiles. At first it seemed ridiculous being able to carry over 100 missiles and 50 Super Missiles. You do need this amount for the battle against Ridley and Mother Brain though.
I found some aspects of Samus' moves to be inconsistent. When she jumps, sometimes she would jump normally, whereas other times she would somersault. To wall jump, you need to somersault towards a wall, then press the direction away from the wall, then the jump button (has to be done as distinct button presses; this doesn't work if you press jump and the direction at the same time). Often I would normal jump towards a wall, or Samus seemed to refuse to want to jump away from it. The inconsistent jumping is frustrating later on when you get the Space Jump, or Screw Attack which allows you to jump infinitely, or attack whilst jumping respectively. The inconsistencies meant I struggled to traverse up the shafts or the long corridors.
Metroid, alongside Castlevania was a huge influence in the "Metroidvania" genre, so there is no denying the impact on the industry. The beginning of the game was incredibly fun to play. However, the game becomes a bit of a slog during the final part which destroys the pacing. It's still a brilliant game, but not perfect like some people suggest.