To close the portal, he must enter Hell again to stop the invasion.

He gets back into the fight and exterminates the hellspawn from the town, and finds another entryway into Hell. Then, the remaining humans discover the source of the hellish invasion: the Marine's hometown. Humanity escapes, and he sits quietly waiting for death, knowing he saved his species. The Marine manages to enter the infested starport, slay all the demons in his way and is able to shut down the force field.

All of humanity's remaining soldiers make a desperate assault on the starport, but eventually they are decimated and only the player remains.

However, the starport is the only way for the ships to depart and the demons have protected it with a force field. With all the major cities in the world in ruins, the remaining leaders plan to use spacecraft to transport the survivors of Earth's population. He discovers that Earth has been invaded (as foreshadowed back in Doom I). The player once again takes the role of a lone Marine, an unnamed hero, who, after escaping Hell and being the only survivor, returns to Earth.