After a series of delivery trucks are robbed and destroyed south of the border, Joyce Manning (Sally Fraser) fears her brother, Lt. Col. Glenn Manning (Dean Parkin), is the culprit. Manning is a gigantic behemoth, a victim of overexposure to radiation who has gone insane due to his freakish size. The giant is eventually apprehended by the Army and held captive. But the frenzied beast is too much for his captors and soon breaks out, intent on laying waste to Los Angeles.