A group of Cultists, a splinter faction of the Marauders led by Adam Hale, attacked and captured a terraformer that controlled the weather of Mars. Red Faction forces, led by Frank Winters along with Darius Mason, made an assault on the terraformer. Unfortunately, Adam, disguised as one of the Red Faction, tricks Darius and destroys the terraformer. The destruction of the terraformer results in catastrophic weather which forces the entire population underground.

A few years later, Darius now works as a mercenary along with a former marauder named Kara. Darius was sent to do a mining job and learned that he was tricked again by Adam Hale and the cultists to open a seal that releases an alien race called the Plague, and becomes trapped in the tunnels the Plague live in. After falling off a cliff that left him unconscious for three days, Darius, guided by SAM (Situation Awareness Module) escapes the tunnels into an underground mining colony, to find the Plague attacking the miners. Grouping up with survivors and some Red Faction soldiers, he escorts an ambulance convoy to Bastion, with help of a L.E.O exosuit found along the way. Darius then arrives at Bastion only to find the town overrun by the Plague, with the injured holed up in the center of town. Darius helps out, only to have them turn on him when they found out he is involved with the Plague. He escapes to the surface, as the pursuers following him are attacked. The Red Faction, along with Frank, shows up and arrests him, but are attacked, and Darius joins the Red Faction and fends off the Plague. Informed that Hale is involved with releasing the Plague, Frank informs Darius where the Cultists are hiding, and Darius assaults them in a walker with Kara, and discovers Hale, trying to tame the Plague. Darius is then discovered trying to kill Hale, and tries to escape in a minecart, but Hale attacks in a walker. The walker is crippled by a landslide, and Darius uses his Magnet Gun to drop pieces of the ceiling on him, decapitating Hale. He and Kara escape in the walker to a Marauder city, when they try to learn more about the Plague. They discover a secret entrance that is filled in with lava, so Darius and Frank pilot a ship to a machine used to raise the lava, and destroys it to lower the lava. Darius and Kara then assault the tunnels to find the Queen, but Kara is killed when they get out to repair a leg. Darius then makes a personal vendetta against the Queen. He finds her, and in a fight, badly weakens her, but she starts to escape and destroy Mars. SAM discovers that the Plague cannot withstand pure air, and Darius follows the Queen to the surface, and finds and repairs the terraformer that Hale destroyed, killing all of the Plague.

Red Faction arrives at the terraformer, and SAM warns Darius of high concentration of radiation incoming: sunlight.