Billy "Commander Keen" Blaze, an eight-year-old genius, has flied to Mars in his Bean-with-Bacon Megarocket build out of common household objects. But while Keen was exploring Mars, the alien Vorticons stole vital parts from his ship and hid them in the Martian cities. Now Keen must find the stolen parts if he wants to return to Earth.
Commander Keen 1: Marooned on Mars is the first in a series of platform games. Your objective is to find the 4 missing parts to your ship.
The game begins with a top-down map of Mars. This is the level select screen, where you can walk around and choose the next level you want to enter. On each level, you have to find the exit, and possibly grab a missing rocket part which may be on the level. Once you reach the exit, you're back on the map of Mars.
The levels are typically full of enemy creatures. Most numerous are Yorps, which are mostly harmless, if annoying, but other creatures are a genuine danger. If Keen gets shot or touched, or falls into a pit or some hazardous object, he dies, and you're booted out of the level back to the map of Mars and lose one of the lives.
Thankfully, Keen can defeat some of the enemies with his raygun. He can also find a pogo stick which allows him to jump very high. Other items to find include keycards that open locked doors and bonus items which give score (Keen gets an extra life if he collects enough score).