A space probe returns to Earth from Mars carrying a deadly microorganism that's released when inspected by a nosy train employee, quarantining an entire train station and trapping those who are infected, including government biologist Dr. Sorensen and his colleagues Charlie (Ralph Meeker), Jack Tiller (John F. Goff), Jenny (Carol Irene Newell), and Hank (George "Buck" Flower). Those trapped must stay awake to avoid the deadly virus, which causes the brain to swell and leads to a fatal skull fracture when asleep.