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In this sequel to the original DOOM, the protagonist is still the same hero - the last remaining space marine. After having single-handedly saved Mars from demonic threat, he returns to Earth, only to find out that the demons have already invaded it, killed most of its inhabitants or possessed them. It’s his task to bring down the force field around the last operational star port to allow the remnants of mankind to escape to the stars. DOOM II looks and plays very similarly to its predecessor, utilizing the same 3D graphical engine with 2D sprites for enemies. The gameplay once again consists entirely of navigating the hero from first-person view through 3D environments and shooting at the demons while attempting to find the way out by flipping switches and looking for keys. Unlike in DOOM, which is divided into three episodes, the 30 levels of this game (plus the 2 secret levels) form one long episode. The game adds one new weapon to the player’s arsenal; the super shotgun, several new demon types with more advanced attacks than those of the predecessor, such as the chaingun-toting Heavy Weapon Dudes, the skeletal Revenants who launch homing missiles and the sinister Arch-Viles who have a highly damaging fire attack.
description from MobyGames