Schoolteacher and family man Ed Avery (James Mason), who has been suffering bouts of severe pain and even blackouts, is hospitalized with what is diagnosed as polyarteritis nodosa, a rare inflammation of the arteries. Told by doctors that he probably has only months to live, Ed agrees to an experimental treatment: doses of the hormone cortisone.
Ed makes a remarkable recovery. He returns home to his wife, Lou (Barbara Rush), and their son, Richie (Christopher Olsen). He must keep taking cortisone tablets regularly to prevent a recurrence of his illness. But the "miracle" cure turns into a nightmare when Ed begins to misuse the tablets, causing him to experience wild mood swings and, ultimately, a psychotic episode which threatens the safety of his family.