You will play as a former marine who has been demoted to a janitor and transferred to work on Mars, in the laboratory complex of the UAC corporation. Something went wrong with the experiment involving new teleportation technology, and now the rooms of the Martian base are teeming with monsters and crazed security. The main character is the only survivor who needs to escape from this hell.
However, the game's story is merely a couple of pages in a file and the text at the end of each of the three (or four in the Ultimate edition) episodes. The main thing in Doom is the gameplay, which made the game a breakthrough in the genre of 3D shooters.
Each episode consists of nine levels (one of which is secret and not mandatory to complete). On each level, you need to get from the entrance to the exit (and often press the corresponding switch there). A level is a maze with many paths, doors (including hidden ones), and monsters. Sometimes, to reach the exit, you have to press switches that open passages and look for keycards for locked doors.
For monster extermination, there are as many as 8 types of weapons: brass knuckles, a chainsaw, a pistol, a shotgun, a machine gun, a rocket launcher, a plasma gun, and the overkill weapon BFG-9000. Each of them has its own features when used. For example, brass knuckles and the chainsaw only work in close combat, rockets can damage you if they explode nearby, and the BFG releases additional charges after the main one hits the target.
As you progress through the game, you also need to collect various bonuses. Weapons that you don’t have yet, medkits, a bulletproof vest (which absorbs some damage). In the hiding places (most often located behind doors that look like a regular wall), you can find "super-bonuses" that raise your health and armor level to 200%, as well as a level map. Fortunately, the map in the game can be viewed, but you will only see the places you have been.
Although the game is generally three-dimensional, there are certain nuances. The pistol, machine gun, and shotgun automatically hit enemies within sight, even if they are significantly higher or lower than you. In contrast, rockets and plasma projectiles (as well as various monster "projectiles") have a certain limit for guidance.
The multiplayer mode deserves a separate mention. Doom is one of the first games to feature it. By connecting via a local network, you can complete the game in cooperative mode with friends or fight against each other, earning points for each kill (and losing points for each "suicide"), which in the game are called fragments.
The word "frag" and the name of the Deathmatch mode became well-known gaming terms after the game's release, not least thanks to the incredible popularity of Doom, its sequel, and other id Software games.