While the SAS: Zombie Assault series does not have a traditional plot or storyline, mission descriptions of the game's maps give an idea of the game's overlying plot.

The game begins with members of the elite Special Air Service (SAS) being sent into a rural area after reports of zombie attacks. The mission is to investigate these incidents as well as get rid of any evidence, but while staging an operation in a farmhouse, the SAS become under attack by a horde. After shooting down the zombies and surviving the attack, the members of the S.A.S. have a new goal: investigate and eliminate the source of this new strain of zombies