It begins on the Planet,Zebes where Young Samus was raised & trained by the Chozo after her parents were killed by a Monster. She was infused with Chozo DNA & became a Bounty Hunter.
Later,after taking down the Charge Beam Beast,The Chozo warns Samus about not only the Space Pirates,but some unknown species on Planet SR388. Samus goes there to investigate. She notice the Space Pirate's objective was acquire these life-forms called Metroids. After she got spotted,Samus begun taking them down. The Metroids broke free & drain the energy out of the pirates. Samus has been attacked by the Space Pirate member,Ridley,the monster who killed her parents.
While Samus panted,The Chozo calm her & tells her that fear won't help complete the mission. Samus understands and fought the Space Pirates & Metroids. She then confronts Ridley & battle eachother. She manage to gain the upper hand to the Super Missle into Ridley's mouth,killing him. With her mission complete,Samus goes to her ship & flies into space.