Bounty hunter Samus Aran responds to a distress call from a Space Pirate vessel orbiting Tallon IV, only to find the Pirates conducting illegal biological experiments aboard. The frigate's destruction forces Samus to chase her arch-rival Ridley down to the planet's surface, where she discovers the ruins of an ancient Chozo society and encounters Phazon, a devastating substance spreading corruption throughout the world's ecosystem. As Samus explores the ravaged environment, she must uncover the origins of this toxic force and determine what role the Pirates and Ridley played in Tallon IV's deterioration.