1961. On Valentine's Day, a girl named Susan is visited by a boy named David. They are playing with a steam train, giggling and clearly sympathizing with each other. For their idyll through the window is watching another boy - Harold, who afterwards throws a valentine under Susan's door. But the girl rejects the paper heart with a confession and, with a sneer, crumples it. On that Harold, catching the moment when Susan went to the kitchen, sneaked into the house and killed the rival.
Nineteen years pass. Susan has grown up, has a husband and a child. One day she needs to go to the hospital to get an X-ray she had taken the day before. A man in a doctor's mask sneaks in and tampered with the woman's examination results, and eventually the doctors discover leprosy (living decay) on her X-ray, are frightened and forcibly send her to a special room. Meanwhile, whoever did this starts slaughtering all the medical staff and patients, getting closer and closer to the desperate Susan....