Tessa Sanderson, ex-FBI agent, has moved to a sleepy Maryland town to escape her tragic past. When the town's beloved dogs begin viciously attacking pet owners and their children, federal CDC agents determine that the dogs are carrying a mutated flu affecting the aggression center of their brains, for which there is no known cure. Tessa offers her unofficial assistance to Animal Control Officer Jess Langstrom, who has been ordered to round up all the dogs and quarantine them. Meanwhile, some of the locals, unconvinced of the threat, are preparing to protect their pets by any means necessary. But Tessa, the widow of an Arab who roused the suspicions of her FBI colleagues, has another secret: Someone is sending her threatening e-mails in Arabic that claim responsibility for the virus, and she resolves to go deep undercover to expose a deadly conspiracy.