While lending mission support on planet SR388, interstellar bounty hunter Samus Aran is attacked by an X parasite, an organism with the ability to mimic the abilities of any creature it infects. Near death, Samus is saved by a vaccine made from the DNA of the last Metroid -- the X parasite's only natural predator. When the X spread to a research station in orbit around SR388, a weakened Samus is forced to exterminate them all ... or die trying.