The planet setting of Metroid II: Return of Samus is SR388. The Game Boy adventure sends Samus Aran to the Metroids' homeworld to eradicate the species before the Space Pirates can exploit it.
SR388 is presented as a dangerous world of underground caverns and tunnels. Samus advances by hunting a fixed population of Metroids, whose numbers are tracked by an on-screen detector. When the Metroids in an area are eliminated, an earthquake lowers the planet's lava levels and opens a route deeper into the world.
The game introduced several important series elements, including the Space Jump, Spider Ball, and Spring Ball. Its Metroids also appear in multiple evolutionary stages—Alpha, Gamma, Zeta, and Omega—rather than only as the small floating creatures seen in the original game.
SR388 is sometimes confused with Zebes, the setting of the first Metroid and Super Metroid, or Tallon IV, the principal world of Metroid Prime. The story's ending is especially important to the wider series: Samus spares a newly hatched Metroid, setting up the events of Super Metroid.