You play the role of a genius eight-year-old boy, Billy Blaze, who has managed to construct in his backyard a spaceship made from household objects and cans, called "The Bean-With-Bacon Megarocket".  When his parents are out and the babysitter falls asleep, he wears his brother's Packers helmet and becomes Commander Keen, Defender of Earth.  Keen is exploring on Mars when aliens steal four components from his spaceship that he must get back. The aliens are the Vorticon, a fierce dog-humanoid race that had an outpost on Mars. Keen travels through different martian cities, searching for all the missing parts in order to get home.

As you start the game you get an overlook of the map where you can see your crashed spaceship and the Martian cities you are about to explore and which is, by the way, the only place where you can save the game.  The controls are pretty simple. You can walk left and right on the screen, and jump to get on higher platforms. Some of these platforms are semi-solid and can be jumped through from below. As you find the pogo stick, it gets harder to control Keen, but allows you to get twice as high if you press the jump button at the right moment.  The ray gun (when you find it), can be used to fire slow moving rays straight left or right to kill enemies. Some enemies die after one shot, some after more, and others are resistant to the ray gun. In addition to the ray gun and the pogo stick you can also find during the game:

* Rayguns and Charges: Provide additional shots.
* Score Items: Usually in the form of sugary foods and beverages, these items give Keen varying amounts of points (from 100 to 5000 points). The items worth the most points are generally difficult to access. Collecting enough (20000 points) will get the player another life.
* Colored Keycards: Red, yellow, green, and blue cards open doors with corresponding colors, and are usually necessary to collect to complete a level.

You don't have a health bar so If Keen touches an enemy, he immediately loses a life. Enemies include Martians, and Vorticon. There are also several hazards in the games that will kill you when touched, such as fire and acid.

Overall it is one of the best game series ever. It may have the simple plot of retrieving stuff for your rocket, but it is better than the games that involve you rescuing a damsel-in-distress. This game is ancient, but ID did not throw in some timer problems, and that means that you will not have a problem running the game on modern systems. If you like platform games, especially the ones that involve you walking around a map and traveling to your next destination, then you won't go wrong with this one.