You play Master Chief as you run around the multiple 64 screen levels blasting baddies, and collecting weapons and other goodies.
Check out the Atari Age thread on the creation of the game. There are physical carts as well as a free Rom available.
This is the online emulated version. The arrow keys move and the space bar shoots in this version. You don’t start out with a weapon, so be sure to go up one screen and pick up a gun before you venture too far off to the right-hand screens.
Here is a screen shot of it playing in my Stella emulator: