Ed Avery is an amiable schoolteacher living in suburban America with his wife, Lou, and their young son, Richie. To make ends meet, Ed works a second job as a taxi dispatcher, but his health begins to deteriorate. After collapsing at home, he is rushed to the hospital and diagnosed with a severe, almost terminal case of polyarteritis nodosa. Miraculously, doctors suggest an experimental treatment using the newly-developed corticosteroid drug cortisone, which brings Ed back from the brink of death. As Ed recovers, the medication initially appears to be a miraculous cure, restoring his vigor and enthusiasm for life.

However, Ed's newfound energy starts to take a dark turn as he increases his cortisone dosage without medical supervision. His behavior becomes erratic and authoritarian, causing concern among his friends and family. His moods swing wildly between euphoria and rage, leading to increasingly strained relationships with Lou and Richie. As Ed's mental state deteriorates further, it soon becomes clear that the drug that saved his life also threatens to destroy it, leaving his loved ones in a desperate struggle to cope with his transformation.