People in a nearby city panic when a nuclear missiles goes astray. It detonates harmlessly but a group of scientists take notice of the incident, the fourth near miss since the thermonuclear age began. Dr. Philip Gainer warns his associates that there will inevitably be more incidents, and one will prove fatal. He proposes that they make the nations of the world unite. As they listen to a painful squealing noise from a covered box, Dr. Herschel says that one of them will have to undergo the same ordeal.
Gainer insists that only when there is a common enemy or fear do men unite. He proposes that they create an alien for the nations of Earth to band together against. Herschel asks what happens if something goes wrong, pointing out that experts will be examining their fake alien. Gainer is confident that their plan is perfect and their creation, an alien from the planet Theta, will be as perfect as the animal in the box. The two men remove the covering and open the case, and Gainer removes the altered animal and displays it to the others.
All of the scientists write their names on pieces of paper and put them into a pot. Gainer then chooses the name of the one among them who will volunteer for the process: physicist Allen Leighton.
Allen goes to Gainer's Advanced Biological Studies Group lab and Gainer prepares him for the surgery. He warns Allen that he will undergo intense physical and emotional stress but Allen is seemingly unconcerned. They go to Gainer's office and the doctor prepares to give Allen the first injection. However, Allen's wife Yvette calls to him from outside. Allen tells Gainer to give him the injection and Gainer reluctantly does so.
Once Gainer is does, Allen goes to see his wife in the hallway. They go window-shopping for maternity dresses and assures Allen that she isn't pregnant... yet. However, she tells him that her obstetrician has determined that her heart murmur has faded and it's now safe for her to have children. Allen asks how she would raise a child if something happened to him, but Yvette dismisses the eyes, licks the tip of her finger, and presses it to Allen's forehead in their secret sign warding off evil.
Later, Allen returns to the lab and Gainer begins the next step of the transformation. Gainer places Allen in a hyperbaric change and increases the pressure and nitrogen in the atmosphere. Afterward, Gainer checks Allen's transformation while the creature in the cage squeals. Allen wonders how long it will be until he sounds the same way but Gainer ignores the question and tells him to rest for an hour. Before he goes, Gainer tells Allen to start working out how he will fake his death.
Allen plans a trip and tells Yvette that he has no choice but to go. She wishes that he would take a teaching job but he says that it's too late, and Yvette suggests that he and his scientists work on problems on Earth rather than exploring other worlds. Allen wonders why she brought up the subject but then dismisses his concern. He suggests that what they need is something to come along and scare them so that they would unite and stop fighting. As it continued, they would get used to idea of uniting and enjoy it. Yvette dismisses the idea as silly. Gainer arrives to pick Allen up and Yvette says that she's going to the airport with him. Allen says that she'd have to come home alone and he'd worry, but Yvette insists. He kisses her and gives her the ward against evil, assuring her that everything will be fine.
Gainer and Allen go to meet with Herschel, who is overseeing the construction of the fake Thetan aircraft. They've managed to create a metal similar to what they've detected on Theta to aid in their deception. Herschel explains their plan to launch the capsule and Allen into orbit and then drop him along to a flight pattern that would seem to have originated from Theta. He'll land near the UN while the General Assembly is in session. They tell Allen to master the controls and then return to the lab for another session that night.
Allen undergoes radiation treatment and tells Gainer that he can only sleep for 20 minutes at a time and then has sweats. Gainer tells him to unleash his emotions but Allen insists that there's no point, and then asks if Gainer has prepared the telegram telling Yvette that he's "dead." His associate says that he will do it shortly and then goes out. Yvette arrives with some packages and Gainer gives her the telegram with the fake death notice. He says that it was a secret mission but Yvette doesn't believe that her husband is dead. She insists that she would know it if Allen were dead and breaks into tears. Gainer offers to take her home and Yvette quickly leaves, abandoning her packages. Once she's gone, Allen comes out and discovers that the packages contain clothing for an expectant mother.
Sometime later, the scientists begin extensive surgery to add the new organs and restructure Allen's body. He is already partway into his transformation and begins to pass out. Allen tells Gainer that he can handle it and asks about Yvette. Gainer says that she loved Allen very much and Allen asks his colleague to take care of her. The scientists return to discussing the surgery while Allen's vision begins to blur and his colleagues fade in and out of his vision. He staggers away and begins to rant about flying and mad doctors. Gainer tries to get him on the operating table but Allen knocks him to the floor. He helps his friend up and then hits him again and leaps away.
The scientists try to capture him and tie him down but Allen throws flasks at them, laughing, he frees the animal from its cage and it scurries off into the shadows while Allen continues his rampage. He grabs an x-ray machine and threatens it with them, and then turns it on as they duck behind a counter and then split up. Gainer switches off the lights and Allen goes to the phone and tries to call Yvette. The scientists grab and gag him just as he makes the connection, leaving Yvette to wonder who is calling. Screaming, she demands to know who is there but get no response.
Later, Allen wakes up and Gainer tells him that he had a schizophrenic episode a few days ago. They kept him under sedation until they found the hormonal extract that caused his delusions. Gainer gives him another injection and prepares him for surgery.
As the scientists operate on Allen, Yvette comes to the lab and buzzes at the security door to the operating theater. She tries to go through, insisting that her husband worked there, and Gainer comes out and tells her that she can't keep coming there. Yvette explains that she came there to get Allen's things and got a feeling that her husband is still there. Gainer dismisses her hunches as delusions and tells her to accept that Allen is dead and to think about her child. He goes back into the operating theater and Yvette goes to Allen's office.
The scientists continue the operation but the new organs threaten to shut down. They take Allen off the artificial lung and after a few moments, he starts breathing on his own. Meanwhile, Yvette suffers sympathetic pains and collapses into a chair briefly. As the surgery ends, she recovers, finishes packing Allen's things and leaves the office. She sees Gainer and the others wheel a covered gurney out of the operating theater, and into an elevator. Gainer looks at Yvette for a moment before the door closes.
Gainer and the others take Allen to a room and hook him up to a voicebox so that he can speak. He finally manages to communicate and tells them that the pain has faded but he is suffering from odd sensory impressions. Allen admits that he feels isolated and is dreaming of Yvette. Herschel tells Allen that they'll be flying him to an atoll and send him and the capsule into orbit via a weather satellite. Once in orbit, Allen will be retrorocketed to the designated position and then re-enter Earth's atmosphere. He'll then go to the General Assembly and make it clear that he's an advance scout. They will provide him with a laser weapon for use in an emergency. Allen asks them if the plan can really work and Gainer warns that there are no absolutes. He thinks the chance of success is good and that soldiers have gone to war with much less chance of success. Allen thanks him for the reassurance and they take him out in a coffin.
Soon, Allen is launched into orbit and his capsule descends. Reports of an unknown spacecraft soon fill the airwaves. The authorities have determined that it will land in ten minutes in the vicinity of New York City. Meanwhile, the scientists monitor the capsule's descend and discover that it has gone off-course. They figure that when it lands, Allen will head for the lab and go there to meet him.
Three hunters in the woods hear the spacecraft land nearby and figure it's the UFO they've heard about on TV. They go to investigate and hear something stumbling through the woods. Their dog Ginger runs into the brush and then runs back and away in terror. When the hunters try to go in, they watch as an alien hand aims a laser gun at their station wagon and vaporizes it. The transformed Allen walks away but one of the hunters fires, shooting him in the chest.
In the street, Yvette clutches at her chest in pain as she feels Allen's agony. She hails a taxi and goes to the lab.
As Allen makes his way through the woods to the lab, Yvette goes in first and finds the operating theater door open. The test animal is in its cage and Yvette hears it squealing. She looks in and is shocked at the disfigured beast inside. Allen lurches down the stairs, bleeding, and Yvette backs away from the alien monstrosity as it reaches for her... and then collapses, dying.
Gainer and the others run in and can only watch as the "alien" makes the gesture warding against evil as it dies. Yvette realizes that it's her husband and Gainer tries to explain. Disgusted, she realizes from her earlier discussion what they had planned. Herschel explains that they drew lots and it could have been any of them, but Yvette dismisses it as a trick. The scientists cover Allen away but Yvette kneels at her husband's side. She tells them that they killed Allen for nothing, but Gainer says that if Allen was willing to sacrifice so much, perhaps there are others who would learn from his example and stoop to save themselves.