Aboard the Kurios Space Control Station in Aartis System, an unknown virus has infected most, if not all, humans. At Bay 10, Military Space Shuttle Mantis 12 is unable to leave due to malfunctioning ramps. As a result a team of SAS forces begins to defend the rear of the shuttle. Eventually, the SAS manages to takeoff, but not before being attack by a mutated boss zombie, which, in-game, changes depending on your character level. When the SAS finishes clearing the shuttle, it departs.
Meanwhile, upon hearing of the news about the virus, dubbed the Thera Virus, the president begins to address worried citizens that all will be taken care of. While on TV, an intelligence officer orders the same SAS team from the Aartis system to recover a vaccine on Thera discovered by Combot, an AI designed for emergency defense, in order to stop an outbreak on Aartis, the known planet where the human race resides. Since the president's address to the people, he has also approved of signaling the Jolt Ship in Thera's orbit to fire its WMD to destroy the entire planet, since there were no signs of life at the time. The team has only less 15 minutes to retrieve the vaccine and get out alive before they are destroyed along with Thera. The team of SAS fights their way through a subway station to reach a train, which leads to a lab containing the Vaccine. Once they retrieve the Vaccine, the team manages to shoot their way through a crowded street filled with all sorts of zombies, including another mutated boss zombie(s).
After retrieving the vaccine, the SAS Team boards a dropship and departs. A few moments after the SAS Team departs, the Jolt Ship destroyed the planet, killing all the zombies on it. ComBot then orders the team back to Aartis System since the virus has reached a heavily populated city of Aartis. There, citizens have barricaded themselves in, but won't last long with the hordes of zombies abound. The SAS are then ordered by ComBot to rescue as many survivors as possible and neutralize any zombies in the area, including survivors who become infected. After rescuing any survivors in the area, the SAS are attacked by another boss zombie.
Upon rescuing civilians, the SAS team are soon briefed about Combot suddenly becoming compromised as the Virus has become a digital pathogen. The SAS arrives at a market area as there are pods that seems to be harvesting humans and mechanical parts, all of which contain either zombies or drops. While fighting through the area, the SAS discovers a Purge nest spawning zombies. When they reach the other parking lot, the SAS discover a Minion Egg. Four minion eggs surround a much larger boss egg, and each minion egg releases a large group of powerful zombies when destroyed. When the SAS Team approach the Boss Egg, spikes emerge from the ground beneath their feet in an attempt to impale the SAS Team. After destroying the Boss Egg, a Boss Zombie emerged, which was dispatched by the SAS team.
Soon, the SAS are guided to a mansion overrun by zombies with a VIP inside. After searching various hiding spots in the mansion, they finally locate the VIP. He is quite whiny as he will call out the SAS team for picking up a droppable item he claims belongs to himself. Eventually reaching the landing zone, the SAS and the VIP notice that Zombdroid Servants and Zombdroid Soldiers are attacking the group. If the SAS manages to hold off the Zombdroids, the VIP will say in relief to get out of there. If the SAS manages to hold off the attack, but the VIP's health is half-way down, the VIP will complain how he was "half-dead".
The SAS are then sent on a mission to a hospital without power. The only way to restore its power was through switches throughout underground tunnels. The SAS went through the zombie-infested tunnels and turned on the power, but Combot activated four Automatic turrets and, later, one or more Loaderbots. The SAS must battle these enemies to complete their mission.
Later, the SAS are sent to a city that was partially barricaded. Remaining survivors of Aartis are awaiting extraction, but zombies are swarming too quickly to set up barricades. The SAS must defend the city from zombies swarming in from the places without barricades. The mission consists of eight waves, each one increasing in difficulty. The SAS cannot let more than 25 zombies through the lines of defense. There is a final boss at the end which, if let through, will cause a loss.
Finally, the SAS are sent to Boreas to investigate why communications were lost. They are greeted by some HVM mercenaries who helped defend the recovery station. After defending from a few waves of zombies, the HVM destroy the ambulance the refugees have been placed into. A fight breaks out and after the HVM have been defeated, the facility is overrun, forcing the player to escape in a truck. The player must now defend the truck from attacking hordes of zombies. Eventually, the truck is hit by an avalanche, killing the driver and forcing the player to fight a final boss on foot.