The player controls a marine whose goal is to stop the Timestrikers from invading planet Earth, traveling through many different environments. Despite using 3D models, Chasm was not a true 3D engine, leading to it often being considered a "poor man's Quake". Despite this, the game managed to host several innovative features, the most notable of which being the player's ability to remove individual limbs from enemies.
Mong, according to the game's manual, are described as "The foot soldier with an attitude. This persistent mutant will switch to hand to hand combat if you blow off his shooting arm. Taking off his other arm doesn't slow him down much either, he'll get his kicks in anyway he can." Mong are found throughout all of the levels from the first episode, The CondemNATION, as well as a few levels from episode four, The Stronghold of Evil.