The first game in the Commander Keen series, developed by the team that would eventually become id Software, and originally published as shareware by Apogee Software.
Child genius Billy Blaze, a.k.a. Commander Keen, has built a spaceship out of common household items and visits Mars. While exploring, a sinister race of aliens called Vorticons steals four integral parts of his ship, which he must locate before returning home.
Commander Keen is notable for smooth, side-scrolling graphics and a non-linear level progression which allow players to take multiple paths through the game. In addition, Keen can find and use a Pogo stick, which greatly increases his jumping ability while reducing his air control, and adds depth and challenge to the platformer formula.