After receiving a distress signal from a Space Pirate frigate in orbit, bounty hunter Samus Aran boards the vessel and discovers the Pirates have been conducting biological experiments. When the frigate is destroyed, Samus pursues her nemesis Ridley to the surface of the planet Tallon IV, where she uncovers the remains of the Chozo civilization and the source of a toxic substance called Phazon that is corrupting the planet's ecosystem.