During the Cold War, shortly after the Soviet Union launched Sputnik 1 in October 1957, an object from space crashes in the ocean just off the coast of Maine and then enters the forest near the town of Rockwell. The following night, 11-year-old Hogarth Hughes investigates and finds the object, a 100-foot tall alien robot attempting to eat the transmission lines of an electrical substation. Hogarth eventually befriends the Giant, finding him docile and curious. When he eats railroad tracks in the path of an oncoming train, the train collides with him and derails; Hogarth leads the Giant away from the scene, discovering that he can self-repair. While there, Hogarth shows the Giant comic books chronicling the adventures of Superman.
The incidents lead a xenophobic U.S. government agent named Kent Mansley to Rockwell. He suspects Hogarth’s involvement after talking with him and his widowed mother Annie (Hogarth’s father was a U.S. Air Force pilot who died during the Korean War), and rents a room in their house to keep watch on him. Hogarth evades Kent and moves the Giant to a junkyard owned by beatnik artist Dean McCoppin, who reluctantly agrees to keep him. Hogarth enjoys his time with the Giant, but is compelled to explain “death” after witnessing hunters killing a deer.
That night, Hogarth is interrogated by Kent when he discovers evidence of the Giant after finding a photo of him next to Hogarth and brings a U.S. Army contingent led by General Shannon Rogard to the scrapyard to prove the Giant’s existence, but Dean (having been warned by Hogarth earlier) tricks them by pretending the Giant is one of his art pieces. Angered by the apparent false alarm, Rogard prepares to leave with his forces after berating Kent for his antics. Hogarth then continues to have fun with the Giant by playing with a toy gun, but inadvertently activates the Giant’s defensive system and Dean orders it away for Hogarth’s safety with Hogarth giving chase. Dean realizes the Giant was only acting in self-defense and catches up to Hogarth as they follow the Giant.
The Giant saves two boys falling from a roof when he arrives, winning over the townspeople. Kent jabbers wildly at Rogard to return to Rockwell when he spots the Giant in the town while leaving Rockwell; the Army attacks the Giant after he had picked up Hogarth, forcing the two to flee together. They initially evade the military by using the Giant’s flight system, but the Giant is then shot down and crashes to the ground. Hogarth is knocked unconscious, but the Giant, thinking Hogarth is dead and the military are responsible, transforms into a war machine in a fit of grief and retaliates, forcing its way back to Rockwell. Kent then convinces Rogard to prepare a nuclear missile launch from the USS Nautilus, as conventional weapons prove ineffective. Hogarth then wakes up and returns in time to calm the Giant while Dean clarifies the situation to Rogard. The General is ready to stand down when Kent impulsively orders the missile launch, causing the missile to head towards Rockwell where it will destroy everyone upon impact. Kent attempts to escape, but the Giant stops him and Rogard has him arrested for his actions. In order to save the town, the Giant bids farewell to Hogarth and flies off to intercept the missile. As he soars directly into the path of the missile, the Giant remembers Hogarth’s words “You are who you choose to be”, smiles contentedly and says “Superman” as he collides with the weapon. The missile explodes in the atmosphere, saving Rockwell, its population and the military forces nearby but seemingly at the cost of the Giant himself, leaving Hogarth devastated.
Months later, a memorial of the Giant stands in Rockwell and Dean and Annie begin a relationship. Hogarth is given a package from Rogard, containing a screw from the Giant which is the only remnant found. That night, Hogarth finds the screw trying to move on its own and, remembering the Giant’s ability to self-repair, happily allows the screw to leave. The screw joins many other parts as they converge on the Giant’s head on the Langjökull glacier in Iceland, and the Giant smiles as he begins reassembling himself.