Samus Aran first finds the Metroids on a planet called Zebes in the base of Mother Brain during "Metroid". She also finds many Metroids in the three "Metroid Prime" games including a powerful Metroid called "Metroid Prime", which take place soon after "Metroid". Samus is asked by a group called the Galactic Federation to exterminate the Metroid race on a planet called SR388. Samus kills almost every Metroid including the Metroid Queen, but finds a baby Metroid that hatched when Samus found it, and brings it back to the Galactic Federation (as seen in "Super Metroid"). Samus returns to Zebes in "Super Metroid" after a monster called Ridley captures the baby Metroid and brings it there for his master, Mother Brain. Samus finds more Metroids there, including the baby Metroid which had grown very large, even larger than Samus. It doesn't try to attack her because it thinks Samus is its mom, and when Samus is about to be killed by Mother Brain, the Metroid tries to protect Samus, but is killed by Mother Brain, falling onto Samus and fusing with her. Samus kills Mother Brain and escapes from Zebes right before it explodes. Metroids are extinct after this, and because of this, the X virus that the Metroids were immune to started to come back and infest SR388. The X virus infects its host by entering the body, killing them, and copying their look. Samus is infected by one of these and is almost killed, but someone gives her a shot of Metroid DNA that kills the X virus, but also makes her immune to being killed by them. Samus goes to the BSL Space Station to find that the scientists in it were killed by the virus, and Samus has to explore the ship while also avoiding her old suit that an X virus took control of called SA-X. Samus discovers that the Galactic Federation had been cloning the Metroid, and at the end of the game, Samus encounters an Omega Metroid, the final life cycle of the Metroid species (besides Metroid Queen).