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Zero Mission takes place on the planet Zebes, which is divided into the following major areas:
A typical playthrough will have you exploring the Brinstar, Norfair, Kraid, Crateria, and Ridley areas first. These first sections of the game are pretty open to varying routes and in a lot of cases the route choices are more about personal preference because the timings are really similar.
After defeating Kraid and Ridley you can go to Tourian to defeat Mother Brain. This starts the post-game section commonly referred to as Suitless. No matter what category you're running, suitless is always the same, the only difference being hard mode adds more search lights to the search light rooms. After defeating Charlie you will always receive the Gravity Suit, Plasma Beam, and Space Jump and the rest of the game opens up with the only remaining goal being to defeat Mecha Ridley to finish the game.