Easily one of the best Metroid games I've ever played.
After a small bit of exploring, Samus finds the infant Metroid in an open room, yet things seem amiss, and are. Ridley, the leader of the Space Pirates, appears and has a brawl with Samus. After defeating Samus, Ridley swoops out with the infant Metroid in his clutches. To make things worse, the station is going to self-destruct in 60 seconds. As Samus makes her escape, she again finds herself on the surface of planet Zebes, head of Space Pirate operations. Samus again sets out on a trek to find upgrades for her Power Suit and take on Mother Brain again.
The level set up is considerably different to what was seen in the NES Metroid. You actually have to do a little bit of exploring to find your first item, the Morphing Ball. Levels like Brinstar and Norfair are familiar to Metroid veterans, along with some new areas like Crateria (Zebes' surface), Maridia, which is an oceanic area, as well as a wrecked Space Pirate vessel, corroding in Zebes' atmosphere.
You can expect the good old classic power-ups, such as Missles, the Ice Beam, and the Varia Suit, as well as some new ones, such as the Grappling Beam (allows you to swing over chasms), the Gravity Suit (allows free movement in water), and the Speed Booster (lets you run really fast if you hold down the dash button and run for some time).
You also see the same bosses, Kraid and Ridley, as well as two new ones, which also bar the way into Mother Brain's lair.
So, if you're a Metroid fan, and really like side-scrollers, and have a SNES (or a SNES emulator, like the Snes9x) I suggest taking a look into this game. Of all the Metroid games out there, I think that this is one of my favorites.