The Union Aerospace Corporation has been experimenting with teleportation technology on Mars' moons Phobos and Deimos. After early successes, something goes wrong. It seems the scientists have opened a gateway straight to Hell. Phobos base is overrun with demonic creatures, and the whole of Deimos simply vanishes. A squad of marines is sent to Phobos, but all except one are quickly slaughtered. It falls to the surviving marine to grab some guns and strike back at the demons. id Software's follow-up to their genre-defining Wolfenstein 3D, DOOM is another first person shooter: full-on action seen from the space marine's perspective. Like Wolfenstein, the game consists of distinct episodes, playable in any order. The first episode, Knee-Deep in the Dead, takes place in the Phobos base and is freely available as shareware. The full game continues on Deimos in The Shores of Hell and culminates in Inferno, the final episode which takes place in Hell itself. Each episode consists of nine levels, one of which is a hidden level. The basic objective in each level is simply to reach the exit. Since dozens of enemies stand in the way, the only way to get there is through killing them. Switches and buttons must be pressed to advance at certain points and often color-coded locked doors will block the way - matching keycards or skull keys must be found to pass.