Upon hearing of several recent robberies of food delivery trucks in Mexico, Joyce Manning (Sally Fraser), Army officer Lt. Colonel Glenn Manning's sister, becomes convinced that her brother (Dean Parkin) survived his fall from the Boulder Dam (as seen in ''The Amazing Colossal Man''). Along with Army officer Major Mark Baird (Roger Pace) and scientist Dr. Carmichael (Russ Bender), Joyce goes to Mexico to look for Glenn and finds that he has, in fact, survived, but was left disfigured and nearly mindless by the trauma of his fall. Manning is eventually captured, drugged by the Army, and transported back to the United States. He is able to escape again and goes on a rampage through Los Angeles and Hollywood. He nearly kills a school bus full of children. Joyce reasons with him, and he slowly is brought back to his senses. Now realizing what he has become and what he has done, Manning commits suicide by electrocuting himself on high-voltage power lines near the Griffith Observatory.