In the year "20X5," a dangerous new life form is discovered on the desolate "Planet SR388." Before research scientists can safely bring it back for study, it is stolen by a group of space pirates. This life form, named Metroid, could make a deadly biological weapon if used improperly. After several unsuccessful attempts to infiltrate the pirates' headquarters themselves, the Galaxy Federation send Samus Aran, one of the galaxy's best bounty hunters. Armed with cybernetic armor and an arm-mounted laser rifle, Samus is charged with the task of navigating the maze-like corridors of the base and destroying Mother Brain, the pirates' leader.
I loved my NES growing up through the years, Metroid is one of the best still in play. Most of the great NES shooter games were designed as 2D sidescroller/platformer.