Fun Fact: Metroid is sometimes credited with starting the phenomena of “speed running” through a game, as players wanted to see the multiple endings as quickly as possible.
Metroid is the story of bounty hunter Samus Aran, dispatched to the planet Zebes to retrieve the dangerous, energy-leeching Metroid creatures from the Space Pirates led by Mother Brain. Players had to navigate Samus through the tunnels of Zebes and slowly increase her power by gathering energy tanks and attachments for her powered armor suit until Samus was prepared for the final challenge.
Metroid's design was recursive. Players would guide Samus Aran to a part of the planet Zebes, achieve a goal, and then leave. Once they had found a new piece of equipment which allowed them to further explore that area, they would come back. And so on and so forth. Metroid's environment wrapped around itself enough times that by the end, the player could navigate the entirety of Zebes without needing to glance at a map or look for a landmark, and that was a pretty amazing thing for someone to realize they'd done, especially if they hadn't been a video game player before the NES came out.
Players' minds were blown when Samus was revealed to be a woman at the end of the game.