A little girl, Brook, and his family are on vacation to a Montana camping resort. However, everyone except for Brook is infected with a strange rash. They are sifted to the hospital and the NIH are summoned to Montana.
Connor and Dr. Natalie Durant is puzzled by the disease and has no clue on what they are dealing with. Frank Powell scrubbed the cabin the family dines in, but can't find any real source on what they are dealing with too. As patients from other places start to get sick, Stephen realizes the disease is spreading and needs to be stopped.
Stephen and Natalie check out another patient, a sick boy who has the same rashes, and concluded it's smallpox. Smallpox hasn't been in the States for about 50 years. When the FBI shows up, they also suspect that terrorist may be involved. Meanwhile, a Indian, native to Montana, is found dead and infected. As Stephen and Natalie study the body, they are summoned to trauma as Brook's father is in critical condition. He didn't make it.
The FBI find their suspects and Stephen tags along to the scene. They surrounded a car mobile, presumably the lab where they make the smallpox. But when they captured their suspects, they found nothing in the mobile that is even possible to make smallpox. A dead end.
Then, Brook let Natalie and Stephen on a little secret. Just before her family was infected, her brother found a bracelet from the infected Indian. Her brother touched it, but threw it into the water when Brook asks what did he found. Brook picked it up in the water, which washed away the smallpox. Therefore Brook isn't infected and the mother and father are by coming into contact with her brother. Natalie also did a carbon-14 test on the Indian body and concluded that the body is about 150 years old, and was infected with smallpox. There's only one way for a body to survive this long, frozen among the mountains of Montana.
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