The Galactic Federation has sent bounty hunter Samus Aran to take care of the primary problem of the future: Space Pirates. Led by Ridley and his superior Mother Brain, this criminal organization is in possession of the deadly, parasitic Metroid species, which they mean to breed in order to lay waste to the Galactic Federation. Taking on this daunting task, Samus enters the unknown.
The game controls like many platformers, Samus being able to jump (somersaulting when jumping at an angle) with the 'A' button and fire her (oops! spoiled) gun with 'B'. As she progresses through the game, however, she'll collect power-ups from Chozo statues as well as other upgrades. The individual power-ups include: the ability to morph into a ball, hence the name "Morph Ball"; the Long Beam, which allows Samus to fire her beam at greater lengths; the ability in Morph Ball Mode to lay bombs, which create little blasts that help in exploring tight nooks; High Jump Boots, which give the beam-blasting bounty hunter's jump a higher peak; the Ice Beam, which freezes enemies for a time before killing them; the Varia Suit, which significantly increases Samus's resistance to attacks and changes the color scheme of her suit to boot; the Screw Attack, which lets her destroy enemies simply by somersaulting into them; and finally, the Wave Beam, which is more powerful than the Ice Beam and can fire through walls. The other upgrades include Missile Tanks, which allow Samus to fire powerful missiles at enemies and increase her total capacity, as well as Energy Tanks, which increase her total health by 100 (there are eight energy tanks in all, but Samus can only hold six). There's a very satisfying feeling of progression upon collecting a new power-up and a feeling of power upon fighting enemies with better abilities.
The progression is not solely within the character, as the expansive world must be explored in order to beat the game. Though broken up into areas, it's a very non-linear affair wherein the player has the option of taking different paths and returning to previously explored regions of Zebes. There's generally a straight path that must be taken, but wandering off and discovering hidden areas is usually rewarded. The player must make his or her way through Kraid's and Ridley's respective lairs, surviving the onslaught of Zebian lifeforms and navigating the winding caverns. Once this is done, the final area where Mother Brain resides must be overcome, followed by a quick rush out of the Space Pirates's base. This 2D free-roaming gameplay is probably the title's greatest and most obvious contribution to the world of video gaming: being able to navigate the 2D labyrinth at one's leisure, without a time limit or the game blocking Samus from backtracking to earlier areas, is key to the experience.
The game is leagues more difficult than any later entry in the series. Starting out, Samus has 30 health with a capacity for 99. Damage taken is significant, and the player will need to grind and retrieve bits of energy from defeated enemies. It only takes a handful of seconds to undermine the work of twenty minutes of this, however, and the frustration of maintaining one's health somewhat hinders the feeling of being able to explore freely. By collecting power-ups, challenges will become more easily surmountable, of course. After entering a password to continue an in-progress game, the player will start with only 30 health - warranting some true grinding every time a game is continued in this manner. Future games in the series would lend the player a hand with save and restore points.
As the game is about traversing a dark, unexplored world and single-handedly besting its obstacles, the graphics and music are meant to create a feeling of isolation and eeriness. The sprites for Samus and the enemies are typical NES fare - it's easy to have a general idea of what every player in the story is. The environments are textured acceptably enough, and though the plain black background makes Nintendo R&D1 look very lazy, it does add to the creepy, lonely atmosphere that the game intends to create in the player. The music is extremely effective in its role in achieving said atmosphere, from the first creepy and then sweet Title theme to the heroic Brinstar theme; from the easy-listening of the Norfair theme to the spooky tune accompanying Kraid's lair; from the grating music of Ridley's Lair and the Tourian theme to the quiet yet ominous beeping of the Elevator theme; from the Zebetite theme which oscillates between high and low just as Samus is having a similar struggle in the final battle with Mother Brain, to the intense Escape theme that motivates Samus to get out of Zebes as quickly as possible. The visuals may leave a lot to interpretation, but the music is a definite strong point for the game.
Metroid isn't a perfect game, but it's a groundbreaking one. For all its flaws, its intent is clear: to immerse the player in a dark, mysterious world with all the danger and curiosity that such a mission entails. Later entries in the series would make this formula more enjoyable, and it wouldn't be long before hordes of unrelated platformers cropped up with strangely similar gameplay.