DOOM 64 picks up where all the other DOOM games left off. After tearing through space, hell, and wherever else in the previous games, the Space Marine thought that the threat of the demons was finally past. Of course, one survived, and it is currently reanimating all of the fallen monsters in a last-ditch attempt to revitalize their evil mission. The player's task is to finally rid the universe of all the crazy hellions, from the dead guy with the pistol to the rocket-spewing Cyberdemon.
The game is very different from the original DOOM; it has entirely new sprites for the monsters, new textures, pseudo-3D effects (such as bridges), colored lighting, scrolling skies, and custom scripting which allows for effects such as morphing environments and spawning items. The levels are completely new. There are 32 levels in total, including several challenging "fun levels" which cannot be accessed in normal gameplay.
The enemy roster is similar to Doom II. Though several enemies from Doom II are not included, a new monster has been added: the "nightmare imp", which moves and attacks twice as fast as a regular imp. The end boss of the game is also new. The game features a new weapon, the Unmaker, which can be upgraded with "demon keys" found in secret levels.
The atmosphere of the game is even more grim than the original Doom games, in part due to the new soundtrack: instead of rockin' metal tunes, the music consists of creepy ambient sounds and drones.
The 2020 re-release adds seven new levels, including a six-level episode "The Lost Levels".