In a lab, scientists Dr. Jim Farrell and Dr. Frank Rogers discuss a strange microscopic organism discovered on a Mars probe. Their superiors don’t want anyone to know of the organism’s existence, so it is being moved by train to a Denver, Colorado, storage facility. One member of the Mars team, Dr. Ted Sorensen, will accompany the specimen undercover in order to minimize public attention. However, Farrell and Rogers think it is unsafe and unwise to ship the organism until they have a chance to determine its properties. On the train, Sorensen deflects the prying questions of the drunken engine operator, Hank Willis, who is convinced Sorensen is not a real train conductor, especially after noticing his loaded gun. Later, when Sorensen falls asleep, Hank investigates the cargo. Back at the lab, Farrell and Rogers identify the alien substance as a disease organism. Transmitted by direct contact, the specimen kills lower organisms in minutes, but does not seem to affect test rats. Meanwhile, on the train, Hank pulls out a vial of the alien substance, which falls and cracks open when the train lurches. Although Hank puts it back, he cuts his hand on the broken glass. At the research facility the next morning, the scientists find a test rat has died from brain swelling but they can’t determine the trigger. Meanwhile, the train stops at Moose Point, where Hank greets the switcher Jack Tiller, station master, Charlie Weigel and office bookkeeper Jenny Moore. Noticing Hank’s injured hand, Sorensen asks what happened. Hank lies about being in the cargo compartment but Sorensen discovers the truth and places an ominous call to headquarters. In response, the official informs Sorensen that they will contain the area and all communications so the disease will not spread. Sorensen is ordered to use his gun to keep his companions in line. He explains to the others that he is a biochemist accompanying a potentially dangerous substance; since Hank may have been exposed and the other three have had direct contact with him, they are all to be quarantined at the station indefinitely. Jack runs away in fear but Sorensen shoots him in the arm. When Sorensen reports back, he learns he may also be infected. The official warns that no one can go to a hospital until the scientists know how the organism reacts in humans. Sorensen tries, but fails to calm everyone; while Charlie seems resigned, Jenny is anxious to keep a date she has planned for that night and Jack is bored and restless. Feeling responsible for infecting the others, Hank runs away. Sorensen shoots him in the leg but Hank flees into the woods. When Sorensen explains that the National Guard has been instructed to find Hank, the others realize the severity of their situation. Learning about Hank’s escape, the official commands his staff to ensure that Hank gets no further. The official alerts Farrell and Rogers about the containment breach on the train. The scientists wonder how they can come up with a counteragent when they’re at a loss to determine the nature of the alien organism. Later, Farrell and Rogers discover that the organism attacks the central nervous system but is activated only by unconsciousness: anyone who falls asleep will die. As Sorensen delivers the news to his companions and urges everyone to help each other stay awake until the scientists develop an antidote, a wounded cry resounds outside. Sorensen hopes it is a wild animal but Jack and Charlie suspect the sound comes from a dying Hank. After a long sleepless night, in which Jack propositions Jenny, a helicopter drops off supplies. As the hours wear on, fatigue and tempers rise. Though Jenny expresses romantic interest in Sorensen, he refuses her and she turns to Jack for affection. Meanwhile, Farrell and Rogers discover that the alien virus is vulnerable to a laser beam blast. However, the laser rays would also damage the spinal cord, so it is not an ideal option. The scientists try to find a less destructive solution. Back at the train station, the others suddenly notice that Charlie has fallen asleep. As Sorensen reports Charlie is dead, they all watch Charlie’s head swell, his eyes bulge and his brain burst through the top of his head. In the confusion, Sorensen and Jack fail to notice Jenny as she takes Sorensen’s gun, goes to her car and shoots herself in the head. Meanwhile, the official proceeds with plans to “take care” of the five quarantined in Moose Point, stating that national security has to be his priority. He orders Farrell and Rogers to continue working on a counteragent. The phone rings at the train station, but Jack, who is fatigued, paranoid and grieving over Jenny, destroys the phone before Sorensen can answer. When a helicopter drops off a package that claims to be an antidote, Jack is relieved and takes the medicine right away, but dies instantly. Sorenson realizes that they have been sent cyanide, not a cure as promised, and expresses fury at his superiors’ deception. The next morning, as men in protective hazmat suits approach the train station, Sorensen awakens on the floor and realizes that he hasn’t been infected after all. He is relieved to find he’s alive but before he can relay this, the suited men appear at the window and blast him with laser rays.