Doom II: Hell on Earth, was the released in 1994, and was a direct sequel to the original Doom. This game includes many of the same gameplay elements from the first game, but throws in some new weapons and enemies. It was also the first Doom to be initally sold in stores, unlike the first one which came out as a shareware version on its inital release.
Doom 2 takes place directly after Ultimate Doom, with the space marine returning to Earth after killing the Spider Mastermind. It turns out after all, that armies from hell haven't been actually put down, and have already started their onslaught on the people of Earth.
The people on Earth have been endlessly massacred to the billions until they come up with a plan to build spaceships to evacuate them outside of Earth. This plan backfires, as the demons have prevented them from leaving by keeping a force field that appears as flame and blocking entrance. The onslaught continues throughout the whole world. The Space Marine jumps back into action to help save humanity from going extinct. The Marine, along with his fellow comrades, are up to fighting against the hell-spawn, it turns up false for the humans as the Space Marines' comrades end up dying at the hands of the merciless enemies. The Space Marine is the only one left that stands a chance.
After the Marine managed to enable the starpost of letting the humans escape Earth through spaceships, the humans that are in orbit manage to send a message to the Marine and inform him that the base that is spawning the demons is coming from his home town. After fighting endlessly, the space marine manages to go to the other side and deactivate the portal that lets the demons come through.
Fighting long and hard, the Space Marine manages to get to the Icon of Sin (a giant demonic goat-shaped face that spawns the monsters) and destroy it (the main antagonist turns out John Romero's head on a stick), along with the portal as well. Earth is saved from further destruction for now, and the Space Marine along with the few surviving humans left, are put up to the task to bring back life to Earth.
The gameplay follows the same formula as the original Doom. The game was more well-known for its non-linear stages compared to the original, and doesn't really offer the Episode style formula.