Doctor Wesley McClaren is an immunologist in Ennis, Montana, and he has turned down an offer to join his friend Richard Bach, who lives in Atlanta, Georgia, and is the head of a unit called the Biological Response Team (BRT). Wesley and his daughter Holly live on a farm in Ennis, and a farmhand named Frank lives with them. Just over the hill from the farm, a 52 day standoff against militia leader Floyd Chisolm has ended. Floyd has decided to be his own attorney in court. Everyone is shocked when Floyd spits on Judge Tomkins. After the hearing, Tomkins starts feeling sick. Holly, like she always does, stops at Wesley's office after school so she can go home with Wesley. On their way home, Wesley gets a call on his mobile phone. Wesley rushes to the hospital with Holly and tells her to wait in the doctors' lounge, then Wesley is taken to see Tomkins, who looks awful. Wesley is told that Tomkins collapsed in his chambers at the court building. Wesley sees a cop collapse, while Tomkins has a seizure. Wesley calls Richard and tells him to send the BRT. Army medical units are sent in, and Ennis is sealed off. Wesley tells Frank to stay in town instead of going to the farm, then Wesley tells Holly to stay where she is, because the town is dealing with an outbreak. Richard arrives in town and talks to Wesley, who knows that some kind of toxin has been released. Wesley asks Richard about an anti-toxin, then they go to Tomkins's room, where Tomkins has died. Richard tells Wesley that the toxin is a warfare product called NAM37, which is ten times more potent than anthrax. Richard also tells Wesley that a technician working for NAM37's developers stole a big sample of the NAM37 and was detained by security in Billings, Montana, one day after the NAM37 was sold to Floyd, but the technician killed himself before anyone could ask him any questions. After buying the NAM37, Floyd infected himself and started the outbreak by spitting on Tomkins. So far, 94 people have been infected, and the NAM37 has killed 5 people. Wesley takes a sample of Holly's blood, and Floyd is visited in jail by his right hand man and medical expert Nate Pogue, who has also been infected. and the anti-toxin Floyd bought along with the NAM37 only slows the NAM37 down instead of being a cure. Floyd's plan was to start the outbreak, then use the anti-toxin on himself. Floyd tells Pogue to get him out so they can get the anti-toxin that the army has brought into town. Later, a group of men led by Pogue return to the jail, where they kill all of the guards and get Floyd out. Floyd and Pogue go to the hospital with their men, and they take the hospital hostage in hopes of getting the anti-toxin, only to discover that the army's anti-toxin is the same anti-toxin Floyd bought. Richard explains that there have been times when NAM37 has mutated, and when it does, the anti-toxin doesn't work. Pogue sees Holly's blood sample and the sample shows that Holly has been infected, but somehow, she's okay and her body is fighting it off. Floyd wants Holly found. When one of Floyd's men finds Holly in the lounge and tries to grab her, Wesley beats the man up and goes to the farm with Holly and Frank. Floyd's men head to Wesley's farm, where Frank ambushes them and kills all of them except for one man, who kills Frank. Wesley and Holly take off with Frank's body, and ride their horses to a farm where Holly's Grandpa lives, and they give Frank a proper burial. Wesley takes a blood sample from his friend Ann, and just like Holly, Ann's body is fighting off the NAM37. Leaving Holly at her Grandpa's house, Wesley and Ann go to an underground lab that Wesley used to work in, where they plan to come up with a cure. Pogue, who is still at the hospital with Floyd, thinks he has found a cure, but he needs Holly's blood to isolate it. After Wesley finds the reason why Ann and Holly are not being affected by the virus, Wesley and Holly go home...where Holly is kidnapped by Floyd's men. Now Wesley must go to Floyd's headquarters and rescue Holly.