Titles and credits roll over a view of the United Nations General Assembly Building from the East River accompanied by a dramatic piano score. We open with brothers, Dr. Henry Spensser (John Baragrey) and Dr. Jeremy Spensser (Ross Martin) watching an industrial film about factory automation. Sitting on his father's lap is Billy Spensser (Charles Herbert). Jeremy is credited in the film with inventing a heat-sensing device that has revolutionized efficiency in automation. They are interrupted by Jeremy's wife, Anne Spensser (Mala Powers) who reads from a newspaper headline, "International Peace Prize Awarded to Jeremy Spensser." Billy is unimpressed and impatient to see the rest of the movie. The brothers wax philosophical about world peace. Jeremy is convinced, "People of the world get enough to eat, war might just become obsolete." The family travels to Stockholm to collect the prize. They return to New York's Idlewild (now JFK) airport, as reported in the New York newspaper. Jeremy's father, the renowned brain surgeon, Dr. William Spensser (Otto Kruger), is waiting at the airport to greet his family. The proud father is interviewed by the press. He introduces his son's friend, Dr. John Robert Carrington (Robert Hutton). He explains to the reporters, in reply to a question, that the brain of a genius, like his son Jeremy, is no different in appearance to anyone else's. The family clears customs and joins those waiting outside. It is a blustery day, and Billy manages to lose his toy airplane to a gust of wind. He asks his father to retrieve his toy. Distracted by his pursuit of the airplane, Jeremy does not see an approaching truck and is struck and killed. Dr. Carrington pronounces him dead. An ambulance takes the body to the home of the senior Dr. Spensser. Anne remarks, "I don't understand." Dr. John Carrington concurs, "Neither do I." The ambulance crew departs the residence and Dr. Spensser locks himself in his lab. John tells Henry, "He was dead within five seconds." Henry asks, "Then what's all this about an operation? What can he do?" For three hours Dr. Spensser works on his son's body. He opens the lab door, and tells his family, "I did, I did all I could."
At Jeremy's funeral service, his body is displayed in an open casket. He has bandages on his head. John presents the eulogy. Dr. Spensser, angry and upset, exits the service. He asks John if great brains, unhampered by their bodies, could have continued their work to better the human race. John counters that brain and body together are necessary to embody the soul. A brain deprived of a body devolves into a monster. Dr. Spensser almost divulges his secret regarding Jeremy, but thinking better of it admits the discussion is theoretical. John tells Henry that Dr. Spensser will need support in the grieving of his son. Henry reveals his bitterness and sour relationship with his father, "That'll be something new. He's always needed Jeremy before." Gazing out the window at the arrival of Anne and Billy, Henry agrees to John's request for him to stay and support his father. His real reason for staying will become apparent later.
It has been four months since Jeremy's untimely death, and Henry expresses his love for his nephew and sister-in-law. They discuss their lives and the pair plan to go into the city for dinner and a show. Dr. Spensser enters the room, wearing a lab coat, and summons Henry. The two enter the lab, locking the door behind them. Spensser shows his son a tank containing a human brain. Father sarcastically quizzes son on what he sees. It is established that the brain is asleep and can be awakened. Spensser asks the brain a question. He already has the answer. They wait for the brain to complete the computation and respond. A teletype machine gives the answer, and it matches exactly. It finally dawns on Henry whose brain just provided the answer and he is horrified. He tells his father, "No! No, its inhuman." But his father disagrees, "Inhuman. It would have been inhuman to deny the world of his genius." Spensser asks Henry to provide Jeremy's brain a body--a mechanical one. Henry's vanity stroked, he is intrigued by the challenge.
At a dinner party at the house, Anne and John walk the grounds and talk. It is a farewell party for John. When asked about their host, Anne tells John that Dr. Spensser and Henry are engrossed in a secret project. John promises to return. In the lab we see, in shadow, the vessel that will house Jeremy's brain. The phone rings and Henry answers it. It is Anne calling. We see the head and chest of the mechanical man. Spensser is ready to test the machine. Henry tells his father to turn the lever on the left side of the body to activate the creature. A failsafe device, out of reach of the monster's own hands, has been added. He tells his father, "So he won't be able to destroy himself." Spensser turns the level and the brain awakens in the body. He can see and hear. The eyes light up and the voice activates. Jeremy is now aware of his condition and complains he can't move. The Frankensteinish creation clomps around the lab in heavy boots knocking over a table. It sees its' reflection in the mirror and screams in horror. Anne hears the commotion from upstairs and runs down to investigate. The creature collapses to the floor. Anne asks Henry about the noise and the voice she recognizes as her late husband's. Henry dismisses the incident, telling Anne that Dr. Spensser lost his temper at a failed experiment.
Henry returns to the lab. The creature is back in its holding structure. Henry tries to convince his father that to continue Jeremy's new existence is both cruel and wrong. Jeremy implores his father, "You want to help me? Then destroy me!" Spensser bargains with his son's brain, but it demands a condition of his own, "I don't want anyone ever to see me. I will conduct all my experiments in this laboratory with you and Henry and no one else." Spensser agrees to the bargain. They begin their polar plant growth experiments in terrariums. Jeremy complains of seeing images, new and strange ones. One keeps recurring. A ship at sea in the fog, the S.S. Viking crashes. Spensser turns down the power and tells his son to rest.
Convinced the experiment is a failure, Henry discusses a Hawaii vacation with Anne and Billy, but before he can finalize plans, Spensser calls him to the study. They watch a broadcast about a disaster at sea. The pair return to the lab and tell Jeremy he has the power to see events--E.S.P. He confirms he has seen other images. Spensser exits the lab. Henry remains and chuckles, which annoys his disembodied brother. Jeremy threatens Henry, "I warn you, Henry. These are powerful (he presents his hands). You know how powerful because you made them. Don't goad me. I warn you, don't!"
An upcoming meeting is scheduled at the U.N. The topic is the development of polar region grains. John Carrington will be presenting Jeremy's work. It has been one year since Jeremy's death, and the monster plans to visit the grave. His father is opposed, but Jeremy walks towards the door. Spensser tries to stop him, but Jeremy hypnotizes his father with a flashing light from his eyes. He is developing new powers. Jeremy exits the lab and wanders out into the garden. He sees his own headstone and grave marker. He hides himself when he hears his wife's voice warn their son to stay close. Billy places flowers on his father's grave. He is drawn to Jeremy's voice, but recoils slightly at the sight of the giant. Billy asks, "Are you a giant? A real giant? Henry and Anne place flowers on the grave. Jeremy picks Billy up in his arms, and runs off with his son when he hears Anne calling. Anne asks her son about the monstrous thing.
In the lab, Jeremy chastises his father. Apparently Spensser told his son that his wife and son had died. That evening Anne suddenly wakes. She senses something, slips on a robe, and walks out into the garden. Henry discovers her outside and joins her for a walk. She asks about the creature. Henry is cryptic in his response, not wanting Anne to know of his complicity. He tells her they must leave the house, and soon. He kisses Anne. This enrages Jeremy, and he approaches the pair. Anne faints, and Henry disappears. Jeremy picks up Anne and carries her back to her bed. The next morning, Anne wakes crying. Dr. John Carrington arrives to comfort her. She tells him of her encounter the previous night, but he is reluctant to believe such a fantastic story. She insists he leave.
Spensser receives a telephone call. It is Henry calling from a phone booth. He asks for money to get away. Jeremy interrupts his father to confirm that he can see where his brother is located. Jeremy instructs his father to set up a meeting so he can intercept and kill his brother. Henry drives to the site of the appointment. He is expecting a messenger with the money. Jeremy walks the bottom of the East River, ascends the pier and confronts Henry. He kills his brother with a ray that emanates from his eyes. He walks back home. A commotion in the lab downstairs stirs Dr. Spensser. Jeremy is destroying the polar terrariums and tells his father, "Why create food for the maimed and the useless and the sick? Why should we work to preserve slum people of the world when its simpler and wiser to get rid of them instead? Unfortunately there are so called humanitarian scientists, and I am one of them, who tried to keep human trash alive. It will be necessary to get rid of those humanitarians first. You understand?" He hypnotizes his father to champion his plan.
The police visit Spensser regarding Henry's death. They ask about the rumors of the creature. Anne is asked about the stories, but hesitates to be forthright with the police. Anne tells her father in law she has seen the monster, but he tries to convince her she is hallucinating and needs a rest. Billy visits the giant in the garden. Apparently this has been going on for some time. Jeremy becomes upset when Billy reaches towards his on/off switch. Jeremy gives Billy a model airplane, exactly like the one he lost the day his father was killed at the airport. Anne asks Billy about the airplane and is upset learning it was a present from the giant.
Jeremy commands his father to bring Anne and Billy to the U.N. Building at precisely 8:30 p.m. The family drives to the site. John Carrington accompanies them in the car. Jeremy takes the East River underwater route. In the lobby area they gather as Jeremy enters by crashing through a glass partition. The monster dispatches a few people with his death ray. An upset Billy runs to the giant to make him stop. Jeremy continues his killing spree. Billy confronts his father. Jeremy tells his son he can't control himself, but offers to let Billy shut him off. Billy throws the switch deactivating the machine. Jeremy falls over the railing and crashes to the floor below. Anne embraces her son. Dr. Spensser tells John, "Well, you were right Carrington, without a soul there is nothing but monstrousness. I only wish that heaven and Jeremy could forgive me for what I did." We close with a close up of the monster's head dripping blood on the floor.