Space Pirates attack a Galactic Federation-owned space research vessel and seize samples of Metroid creatures. Dangerous floating organisms, Metroids can latch on to any organism and drain its life energy to kill it. The Space Pirates plan to replicate Metroids by exposing them to beta rays and then using them as biological weapons to destroy all living beings that oppose them. While searching for the stolen Metroids, the Galactic Federation locates the Space Pirates' base of operations on the planet Zebes. The Federation assaults the planet, but the Pirates resist, forcing the Federation to retreat. As a last resort, the Federation decides to send a lone bounty hunter to penetrate the Pirates' base and destroy Mother Brain, the mechanical life-form that controls the Space Pirates' fortress and its defenses. Considered the greatest of all bounty hunters, Samus Aran is chosen for the mission. Samus lands on the surface of Zebes and explores the planet, traveling through the planet's caverns. She eventually comes across Kraid, an ally of the Space Pirates, and Ridley, the Space Pirates' commander, and defeats them both. Eventually, Samus finds and destroys Mother Brain, triggering a self-destruct mechanism and forcing Samus to escape the collapsing lair.