The film starts with a young Mexican man driving a truck at a breakneck pace. After the truck ends up submerged in water, the young man gets out of the truck, looking at something and screaming his head off.Sometime afterwards in Guavos, Mexico, the owner of the truck named John Swanson goes into the Sheriff's office to report the truck to be missing. The Sheriff then explains to John that they have already located the young man who was driving the truck, but he is in a state of shock, unable to say anything. Going to the scene where the man was found, the Swanson and the Sheriff note that the trucks tracks indicate that something lifted it off the ground, to which the truck owner finds this hard to believe.The news of the Swanson's disappearing truck soon appears on news outlets. One person who watches the program is Joyce Manning. She soon contacts Swanson, as well as the Army, who send US Army Major Baird to her place. Joyce then tells the two of her theory, that Swanson's truck was taken by her brother, Glenn Manning. Previously, exposure to an atomic bomb had caused Glenn to grow over 60 ft tall. He then went on a rampage that ended with him being shot and plunging into the waters beneath the Hoover Dam. Joyce surmises that the waters may have carried him down to Mexico.Joyce soon makes her way down to Guavos, where the young man comes out of his shock, and explains that it was a "big person" that attacked his truck. Major Baird soon comes down when Joyce tells him what the young man saw. The Major is accompanied by Dr Carmichael. Joyce and several other people go out to the site where the young man was found, and find a large footprint. There are more prints leading up into the nearby mountains. Following them, the group finds an area littered with trucks. However, as darkness is setting in, the group returns back to town.Even so, Joyce is upset when at dinner, Major Baird explains that he has contacted the Mexican Government to supply them with artillery and tanks to capture Glen. Joyce feels that if they can find him, maybe they can reason with him to give himself up peacefully. At Joyce's insistence, Major Baird returns with her to the mountains where some of the trucks were. Here, they find Swanson's missing truck, as well as others that have been emptied, leading the two to surmise that Glen may be raiding the trucks for food. As if to answer their question, Glen appears, though grunting like an animal, and looking through another truck. Half his face is exposed skeleton now due to the fall and damage.In order to capture Glen, the group fills a bread truck with drugged bread. Glen catches the truck and then after eating the contents, collapses due to the drugs. Major Baird, Joyce and several others attempt to bring Glen back to the United States, but there is confusion as to what to do with him. The air transport runs low on fuel, and is forced to land in Los Angeles. Against the wishes of the city, Glen is taken to a hanger at the Los Angeles Airport, and tied down with chains. He eventually escapes, causing some bloodshed and general disarray, before he is tranquilized, and then tied down once again, this time with reinforced chains and a patrol on duty to make sure he doesn't escape again.Dr Carmichael and several others note that Glen's mental condition has deteriorated, and attempt to see if he may have any recollection of who he is. However, their tests illicit no vocal response from Glen but grunts and anguished cries. Due to Glen no longer seeming to be fit to live among human society, he is scheduled to be transported to a deserted island, even though Joyce protests this.While Joyce is with Major Baird, a call comes that Glen has escaped and has headed towards the Griffith Park Observatory. Glen is spotted and picks up a bus full of schoolchildren. Joyce appears and manages to get Glen to put the bus down. Pleading with her brother to think and remember, Glen looks at his sister and says her name, ("Joyce!"), before going to a nearby power line, electrocuting himself to death.