Jeremy Spensser, genius humanitarian, is killed in an accident just after winning the Nobel Peace Prize. His father William, a brilliant brain surgeon, works on the body in secret before burial; later revealing to his other son Henry that he has the brain on life support and hopes to encase it in a robot body! The resulting being is large, strong, and develops many strange powers. Initially it has Jeremy's gentle personality but this, too, begins to change, and a year later it decides to end its long seclusion... Unusual piano music score.

In New York City, scientist and inventor Dr. Henry Spensser, along with his brother, Dr. Jeremy Spensser, and his young nephew Billy, watches an industrial film about Henry's latest creation, a heat-seeking detector currently being used in food manufacturing. Though Jeremy congratulates his brother on the invention, Henry points out that the detector was originally Jeremy's idea. Anne, Jeremy's wife, then bursts into the room to inform her husband that he has just been awarded an international peace prize for his work in developing frost-resistant plants while heading up the World Food Organization. Following his return from Stockholm, where he received the peace prize, Jeremy and his family are greeted at the airport by his father, noted brain surgeon and anatomist Dr. William Everett Spensser, as well as his close friend and fellow scientist, Prof. John Carrington. As they make their way to the airport's parking lot, a gust of wind blows Billy's toy airplane out of his hand, and Jeremy is killed by a truck driver while trying to retrieve the plane. Rather than having his son's body driven to the morgue, William insists that it be taken to his home, where he secretly removes Jeremy's brain from his dead body. After the funeral, William tells John that the brain of a genius like Jeremy should be seen as independent from the rest of the body, while John argues that the brain is just one part of man, that it needs to feel emotions to remain human. Devoid of such impulses, John states, the brain would become dehumanized in monstrous proportions. Though never close to William, Henry agrees to stay on at his father's home, in order to look after him, as well as Anne and Billy, who have moved in with William following Jeremy's "death." After weeks of experimentation, William allows Henry into his laboratory, where he has kept Jeremy's brain alive. William then calls upon Henry, an expert in automation, to create a mechanical body to house the brain. On the night of John's farewell party, the Spenssers finish their creation, an eight-foot colossus. Seeing himself in a mirror, however, Jeremy collapses on the floor in agony. Anne then rushes down to the lab, thinking that she heard Jeremy's cries, but Henry convinces her that it was merely William, upset at the failure of their latest experiment. Back in the laboratory, Henry warns his father that Jeremy's brain could become psychotic if it suffers a second such episode, but William insists that he can convince Jeremy to continue his work, despite the loss of his human life. Though he initially wants to be destroyed, Jeremy agrees to continue his experiments to end world hunger as long as no one aside from his father and brother know of his existence. While working with his father, Jeremy develops extra-sensory perception, seeing an accident between two ships in the Atlantic Ocean hundreds of miles away, though touch and smell continue to elude him. After one year of continuous work in William's laboratory, Jeremy decides to make a pilgrimage to his grave, despite the objections of his father. There, he sees his young son, and Billy immediately befriends "the giant." Furious with his father, who had told Jeremy that Anne and Billy were also killed in the automobile accident that claimed his life, Jeremy destroys the remote control box that William and Henry use to control his mechanical body when he becomes overly excited. Later, Jeremy overhears the love-sick Henry asking Anne to run away with him to Hawaii, and although she refuses, Jeremy become insanely jealous of his brother. The next morning, Anne calls John and the professor rushes over to the Spensser estate, but does not believe her tales of the colossus. Meanwhile, Henry telephones his father from a phone booth and asks for money so he can escape Jeremy. The psychic colossus, however, knows this and orders William to arrange a meeting with Henry by the East River. There, Jeremy murders his brother with laser beams from his x-ray eyes. Returning to his father's lab, the now deranged Jeremy announces that he is turning his back on humanity, arguing that the weak should be destroyed, not saved, and such extermination should begin with all humanitarians. Now under Jeremy's hypnotic control, William denies the existence of any "monster" when the police investigate Henry's death, and tells Anne to stay away from Billy except at bedtime. While putting her son to bed, however, Anne discovers a toy plane, which Billy innocently tells her was given to him by "Mr. Giant," who had recently asked the young lad to start calling him "Daddy." Soon thereafter, Jeremy orders William to take Anne and Billy to a United Nations conference being held in his honor. Hosted by John, the meeting is disrupted when Jeremy bursts through a plate-glass window and begins killing people with his x-ray eyes. Billy then rushes up to the colossus and tells him that he is bad. Realizing that he has truly lost his humanity, Jeremy asks Billy to turn off the power switch hidden beneath his left arm. The young boy does so, and the colossus collapses to the floor, dead. William then tells John he was right, that a brain without a soul is nothing but monstrous.