In an unspecified future, the United States' borders have receded behind a salt line - a ring of scorched earth that protects its citizens from deadly disease-carrying ticks. Those within the zone live safe lives in a society controlled by a common fear. Few have any reason to venture out of zone, except for the adrenaline junkies who pay a fortune to tour what's left of nature. Once out of zone, this group finds itself at the mercy of deadly ticks - and at the centre of a murderous plot. How far are they are willing to go to get to the right side of the salt line?