Doctor Leslie Coburn arrives at her office and deals with her first patient of the day. The second is more challenging: Harry Raditch, a hypochondriac who has been there many times before and insists on seeing Leslie. In the examination room, he insists that he has an actual illness for once. He shows Leslie his left eyes, which is bleeding internally, and then cuts himself to demonstrate that his blood isn't clotting. Finally he bares his chest and shows her ominous black lumps growing on his skin.
Leslie confers with her partner, Dr. Marks, who believes that Harry is just manifesting psychosomatic symptoms. He leaves for an AMA luncheon while Leslie runs more tests. She tries to reassures Harry that he'll be fine but he doesn't believe it. The man admits that he rarely leaves his home any more and his housekeeper does all of the shopping. Harry then realizes that his housekeeper could have the same disease. Leslie tries to calm him down, saying it's most likely an allergic reaction. As she leaves, Harry admits that whatever else, her optimism is contagious.
Outside, Leslie confers with her nurse, Rhonda. As they send his test results to the CDC, one of the nurses points out that Harry is a book dealer. Leslie asks Harry if he's received any books from overseas recently. Harry, embarrassed says he has the book responsible but was worried she wouldn't believe him. He shows Leslie an old science fiction novel. In the story, the crew comes down with the same disease that Harry has. He insists that the disease is from the novel and that aliens brought it to Earth. Worse, in the novel the victims die as their bodies purge themselves of blood, and there's no cure.
Leslie doesn't believe that it's possible and angrily leaves. Outside, Harry's housekeeper arrives... her body voiding itself of blood. Realizing that Harry's contagious, Leslie pulls back her sleeve and sees the same black bumps on her arm.
The CDC puts the medical building under quarantine and sends men wearing protective suits in to run test. Leslie calls Dr. Marks to let him know what's going. She then talks to the CDC representative and confirms that Harry isn't getting any worse, meaning that he's a carrier. Leslie talks to Harry, who explains that he was reading the story and became obsessed with the disease. He couldn't get it out of his mind. As Leslie leaves, Harry asks for her forgiveness for putting her life at risk.
The disease starts to spread, infecting everyone in the building. With time running out, Leslie gets an idea. She figures that Harry's imagination became so powerful that he literally imagined a disease into existence. Leslie sees an article about a meteor shower and suggests they get Harry to imagine a cure.
After making arrangements with Dr. Marks and the CDC, Leslie goes to see Harry. She explains that one meteor from the meteor shower survived and landed in an uninhabited section of countryside. Harry worries that a similar meteor crash caused the Ice Age, wiping out the dinosaurs. Leslie has the CDC bring in a liquid which she claims is from the meteor, and says they have to inject it into Harry to see if the vaccine works. .Harry worries it could make things worse but Leslie convinces him to receive the injection.
Waiting for Harry's condition to change, Leslie dozes off as the staff and patients get progressively worse. Harry wakes her up a little later and reveals that the black lumps have disappeared and his eye is back to normal. Leslie checks the other patients and discovers that they've been cured as well. She thinks it's worked but Harry isn't so sure. He's also now obsessed with the meteor crash. Leslie tells him to forget it but Harry warns that he can't.
Leslie tries to call Dr. Marks but doesn't get an answer. She goes outside only to discover that the entire city, the entire country, the entire world, is now covered in ice: Harry's imagination made the meteor strike true.