Assault

In the year 3104, aboard the Kurios space control station in the Thera system, an unknown virus has infected most, if not all, of the humans. In bay 10, the military space shuttle Mantis 12 is unable to take off due to a malfunctioning ramps. As a result, a team of SAS forces begin defending the rear of the shuttle. Eventually, the SAS manage to take off, but not before being attacked by a mutated zombie boss, which in-game changes depending on your character's level. When the SAS finish clearing the shuttle, they leave.

Meanwhile, upon hearing the news about the virus, dubbed the Thera Virus, the President begins to address the concerned citizens that they will all be taken care of. While on TV, an intelligence officer orders the same SAS team from the Aartis system to retrieve a vaccine on Thera discovered by Combot, an AI designed for emergency defense, in order to stop an outbreak on Aartis, one of the known planets where the human race resides. Since the President's address to the people, he has also approved the Jolt Ship's signal in orbit of Thera to fire its weapons of mass destruction to destroy the entire planet, as there were no signs of life at the time. The team has only less than 15 minutes to retrieve the vaccine and get out alive before they are destroyed along with Thera. The SAS team fights their way through a subway station to reach a train, which leads to a laboratory 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 zombie boss(es).
While rescuing the civilians, the SAS team soon learns that Combot has suddenly been compromised as the Virus has become a digital pathogen.

"The Federation uses stormtroopers when speed, surprise and violence of action are required above all else."
"In addition to his lethal SAS training, a doctor employs a number of quick and efficient means to repair human tissue."
"The special ability of heavy gunners makes them ideal for delivering overwhelming force to enemy positions."

Each class has 7 special skills unique to its type. Ultimate skills can only be trained after level 20. The Assault's special skill is "Assault Team", which summons a team of several soldiers to enter the game and fight for several seconds. The Medic's special skill is "Zombie Antidote", which deals heavy damage to zombies in a large area. The Heavy's special skill is "Air Bombardment", which is an air strike on a targeted area.

When you first create a character, you receive 2000 SAS cash to buy upgrades for weapons, turrets, or grenades. You receive a safe every time you level up, which gives you new equipment or a new weapon. You also receive more money. Another difference is that players now collide with zombies and cannot run through them. This may seem harmless, but it actually fundamentally changes the game, making fighting zombies more skill-based and also making the game more tactical.