CREEPINGNET'S WORLD
METROID
Metroid is one of the early NES platformer titles, released in 1986. The storyline is Samus Aran is on a miission on Planet Zebes and needs to save the galaxy from the evil Motherbrain. It's a side scrolling action platformer that is non-linear. Instead of something like Mario, where you travel to the end of a level, reach a goal, and come back, this is more like Mario II or III if it had a CRPG like non-linearity. Basically, there's a lot of back tracking, hunting for hidden power ups, and travelling around.

Metroid was one of the original classic titles that managed to last to the end o f the NES's lifespan, and is one of the better known properties, if not for the game itself, but for the game's titular villan, Motherbrain, being the main villan on the Nintendo 30-minute long advertisement/cartoon - Captain N: The Game Master, where video game characters from various Nintendo NES games were the cast. It's other major claim to fame is a secret passcode that allows you to reveal the true identity of Samus....in a PG-13-esque way (Swimsuit Clad Samus revealing Samus is a woman).

Metroid is a lot like Kid Icarus or Castlevania, it tends to comoe and go over the years, finding favor for a period of time, then dissappearing into the background again for awhile. It's one of those NES games that has a fair amount of respect but is neither celebrated or talked about as much as say Mario or Zelda, or even Kirby in some circles.
Captain M: The Moron Master - My Experiences
Metroid was one of those games I'd only ever heard about through Captain N, and never played. I ended up buying it with a few other sealed NES games from Wal-Mart in 1988 for about $20 - yeah, mine's the later release with the yellow label.

..and honestly, it's a good one. I used to play this for hours on end, not even caring beat it, just enjoying exploring Zebes as a teenager, finding new areas, glitching through walls, finding shortcuts. I mean, this game was pretty amazing for it's time. Also, the music is pretty rockin'. I did my own version of the Electric Zone theme in 2003 that I still post to this day in my cover music project.