BioTox is a biological weapon that has caused most of the problems with the already current zombie epidemic in the SAS: Zombie Assault series. In SAS: Zombie Assault 3, BioTox is being sold as a "last resort" drug to customers through the black market, further causing the infection to spread across the globe. BioTox can only be taken through a hypodermic needle, meaning those who were already infected were either forced to take the injection or bought the injections through the black market to be reincarnated in the afterlife.
BioTox was released in all parts of Europe. The Military Dominance, crippled from SAS: Zombie Assault 2, has made quarantining the infected nearly impossible. However, the Heroes burnt down the castle at Black Isle, effectively destroying the source of the problem.
BioTox destroyed most of the population of the New England area during the events of the previous outbreak. The SAS is ordered to enter an abandoned office building and secure any info available relating to the substance. The source of the outbreak on U.S. soil was never found, but forensic info suggests that a run-down housing development may be where it started. However, the team has been overrun and must get out of there. The SAS must cover for them. Intel has leaked regarding experiments going on at an abandoned prison psych ward. The SAS must "be ready for anything" in the facility. A rather shocking turn of events takes place when information is found of an attack on the Boston Civic Center, where, at the time, a conference of military weapons providers was being held. When the SAS arrived, though, it was already too late. All attendees have turned. The SAS must now eradicate the infected. Some time later, reports of a robbery are picked up at a shopping mall. It turns out that it is actually another nest of zombies splitting open from an underground bunker. The F.B.I. denies ownership of the facility, but it is possible that they are lying. The SAS must clear out the "store" so a team of investigators can find out what is going on down there.
BioTox experimentation was being conducted at a Russian science institute, however the experiments turned against them. The scientists buried the results in a series of catacombs. However, the zombies have escaped, and taken over an abandoned military base and a private archaeological dig.
At an old Pagoda, an "ancient sect" purposely caused an outbreak with BioTox obtained via the black market. Meanwhile, the zombies appear to have been experimented on by the same apocalypse cult in an abandoned factory. Intel from MI6 claims that the sect is working on creating cyborgs out of the zombies. A scientist is hiding out in a bamboo forest and has access to an antidote. After the SAS rescues the scientist, he must go to a tower that is overrun with zombies. A demolitions team is about to destroy the building, and the SAS is trapped on the roof with hundreds of zombies coming up from below. A rescue helicopter is coming to get him out of the building before it's too late, but if he doesn't kill all the zombies, it may be too late.