After the original Metroid, the series did not see a new release until 5 years later when Metroid II: Return of Samus was launched for the original Game Boy. Following a more linear game design than either its predecessor or sequel, Metroid II was considered to be a weaker entry into the series and is arguably the least popular core Metroid title.
And yet, Metroid II remains the most influential game in the series, contributing not only new elements destined to reappear in later releases, but laying the foundation for Metroid's longest running meta-plot. Metroid II is the least popular Metroid, but its legacy is the farthest reaching.
Sometime after Samus blew the bajeezus out of the Space Pirate base on Zebes, it was decided by the Galactic Federation that the Metroid species was simply too dangerous to be left alone. In a moment of extreme anti-environmentalism, they contracted Samus to head to the Metroid homeworld, SR388, and exterminate them. Once Samus arrived planetside, she set out to accomplish exactly that by traversing a mostly linear series of bland, nearly identical looking tunnels. I suppose there was only so much that could be done with a black and white palette. However, while the limitations of the Game Boy's graphical capabilities resulted in a dull looking environment, those very same limitations inspired a design choice that has become iconic to the series and the character of Samus Aran. I speak of course, of the Varia Suit.
N-Sider.com: Looking back at Metroid: Legacy of a baby Metroid
Threaded Mode