Samus retells the story of the first two games... she leaves the baby metroid behind with the scientists, then suddenly that space station gets attacked, and Ridley kidnaps the baby metroid... the space colony activates its self-destruct as a safety against this, but I guess it failed... then she flies away to Planet Zebes... starting off at Crateria.
Crateria part 1
right now there's only one path and it's... holy shit... it's literally Metroid 1 going backwards... I go down through a blue door on the ground below me... and that leads to the elevator that you ended the game at in Metroid 1, you go in through the busted Mother Brain tube and empty tourian (just a bunch of tiny lifeforms that dont do anything to Samus so far)
but then... at the end of the mother brain hall is just an elevator leading directly into brinstar...