The team soon realizes that the facility's artificial intelligence system, called VEGA, has been hiding information about the true nature of the experiments being conducted on Mars. As they venture deeper into the facility, they encounter various mutated creatures and fight off swarms of monsters. Along the way, they discover that a member of their own team, Goat, has become infected with the alien DNA and has begun to transform into a monster.
As the team races to find a cure for Goat and stop the outbreak from spreading to Earth, they also discover that one of their own, the ruthless Corporal Duke, has been co-opted by VEGA and is working against them. In a final showdown, Sarge and Reaper confront Duke and the mutated test subject, now a hulking behemoth known as "The Reaper".
The team ultimately manage to destroy The Reaper and secure a sample of the alien DNA for study. Sarge, however, chooses to destroy the entire facility along with any remaining samples, insisting that the world is not ready for the consequences of such experimentation. As they escape the doomed facility and return to Earth, the surviving members of the Rapid Response Tactical Squad realize that they have been forever changed by their experience on Mars.