The story follows a Marine who arrives at a U.S. Mars base and meets C.J. The Marine is briefed by Major Tyrone about a missing veteran Marine who disappeared while patrolling the eastern perimeter. The Marine is tasked with reopening a dimensional portal to retrieve the missing soldier. However, things go awry, and the base is invaded by creatures from another dimension. The Marine fights to survive, and along the way, encounters Sergeant Doomguy, who helps the Marine in his mission. Together, they kill the Cyberdemon and close the portal. The story ends with the Marine being returned to Earth, but with a twist, as the creatures from the other dimension are now friendly and helping humanity.
Here are the key facts from the text:
1. The scene is set on Mars, in a US-colonized base.
2. The protagonist is a marine who has just arrived on Mars.
3. The marine meets another marine named CJ, who briefs him on the situation.
4. A Major named Tyrone explains that a veteran marine has disappeared while patrolling the eastern perimeter of the facility.
5. The Major shows a security tape of the missing marine looking into a dimensional portal before vanishing.
6. The mission is to get the portal open again and rescue the missing marine.
7. The protagonist and other marines go to the briefing room to prepare for the mission.
8. The scientist in charge of the operation, Dr. Samuel Hayden, is introduced.
9. The marines discover that the scientist has been lying to them and plans to unleash creatures from another dimension to destroy the base.
10. The marines fight against the creatures and try to stop the scientist's plan.
11. Sergeant Doomguy, a legendary marine, appears and helps the protagonist fight against the Cyberdemon.
12. The protagonist and Sergeant Doomguy succeed in killing the Cyberdemon and closing the portal.
13. The protagonist is rescued and taken back to Earth, where he is greeted by friendly creatures that were previously enemies.