Doom is a 1993 first-person shooter (FPS) game developed by id Software for MS-DOS. Players assume the role of a space marine, popularly known as Doomguy, fighting his way through hordes of invading demons from Hell. The first episode, comprising nine levels, was distributed freely as shareware and played by an estimated 15–20 million people within two years; the full game, with two further episodes, was sold via mail order. An updated version with an additional episode and more difficult levels, The Ultimate Doom, was released in 1995 and sold at retail.
Doom is one of the most significant games in video game history, frequently cited as one of the greatest games ever made. Along with its predecessor Wolfenstein 3D, it helped define the FPS genre and inspired numerous similar games, often called Doom clones. It pioneered online distribution and technologies including 3D graphics, networked multiplayer gaming, and support for custom modifications via packaged WAD files. Its graphic violence and hellish imagery drew controversy.
Doom has been ported to numerous platforms. The Doom franchise continued with Doom II: Hell on Earth (1994) and expansion packs including Master Levels for Doom II (1995). The source code was released in 1997 under a proprietary license, and then later in 1999 under the GNU General Public License v2.0 or later. Doom 3, a horror game built with the id Tech 4 engine, was released in 2004, followed by a 2005 Doom film. id returned to the fast-paced action of the classic games with the 2016 game Doom and the 2020 sequel Doom Eternal.
Doom is divided into three episodes: "Knee-Deep in the Dead", "The Shores of Hell", and "Inferno". A fourth episode, "Thy Flesh Consumed", was added in an expanded version of the game, The Ultimate Doom, released in 1995, two years later and one year after Doom II. The campaign contains very few plot elements, with the minimal story instead given in the instruction manual and in short text segues between episodes.
In the future, the player character (an unnamed space marine, known as "the Doom marine" or simply "Doom guy") has been punitively posted to Mars after assaulting a superior officer, who ordered his unit to fire on civilians. The space marines act as security for the Union Aerospace Corporation's radioactive waste facilities, which are used by the military to perform secret experiments with teleportation by creating gateways between the two moons of Mars, Phobos and Deimos. Three years after Doom guy's posting to Mars, Deimos disappears entirely and "something fraggin' evil" starts pouring out of the teleporter gateways, killing or possessing all personnel on Phobos. The Martian marine unit is dispatched to investigate, with Doom guy left to guard the perimeter with only a pistol while the rest of the group proceeds inside the base. Doom guy listens to the sound of combat on the radio for hours until there is nothing but silence. With no way off the moon and wanting to avenge his friends, Doom guy decides to fight his way through the complexes of the moon base.
As the last man standing, Doom guy fights through the onslaught of demonic enemies to keep them from attacking Earth. In "Knee-Deep in the Dead", Doom guy fights through the high-tech military bases, power plants, computer centers and geological anomalies on Phobos. It ends with Doom guy defeating the two powerful Barons Of Hell (popularly known as the Bruiser Brothers) guarding the teleporter to the Deimos base, and thereupon entering the teleporter leading to Deimos, only to be overwhelmed by monsters in a seemingly unwinnable battle on the other side. In "The Shores of Hell" Doom guy awakens with only the pistol once again and fights through installations on Deimos, similar to those on Phobos, but warped and distorted from the demon invasion and interwoven with beastly architecture. After defeating the titanic Cyberdemon, the Doom marine crawls to the edge of Deimos, which has been flattened into a pancake shape, only to discover that the vanished moon is floating above Hell. "Inferno" begins after the marine rappels off of Deimos to the surface of Hell. The Doom marine battles his way through Hell, ultimately finding and slaying the Spider Mastermind that had masterminded the invasion of Phobos and Deimos. A hidden doorway back to Earth opens for the hero, who has "proven too tough for Hell to contain". The Doom marine, however, is greeted by the grisly sight of a burning city and a rabbit's head impaled on a stake (named in The Ultimate Doom as the Doom marine's pet rabbit, Daisy), showing that the demons have invaded Earth. In "Thy Flesh Consumed", the Doom marine fights the demons on Earth through a variety of disconnected high-tech bases and demonic temples, though ultimately the forces of Hell are not defeated in their invasion of Earth, setting the stage for Doom II: Hell on Earth.