The killer is armed with a lethal weapon: an invisible pathogen that has been released into the Washington Metro. Two hundred people exhibit symptoms, but only three are pronounced dead. Dr Samantha Owen is the forensic pathologist called to consult on the case, but as she dissects the mysterious connections between the victims it becomes clear that this attack is not random. This pathogen is targeted. As Sam starts to close in on the killer's identity, the further she propels herself into danger. Finding the truth might just lead Sam into risking her life...