At the height of the cold war, with superpower threatening superpower with total destruction by nuclear weapons, a meeting of scientists takes place to discuss the future of mankind, following another "near miss" involving a rocket carrying a warhead.
Leading the discussion is Dr. Phillip Gainer. His words to the panel are cold and immediate. Something must be done to distract the world's governments from each other before it's too late and the world is destroyed. Gainer points to history, noting that the only time men have stopped fighting each other, is when faced with a common fear or enemy, and that this is the answer to their current dilemma. He postulates that if all mankind is suddenly threatened by an alien creature from another planet, the nations' governments would stop their aggressions toward each other and join together as one to fight this common enemy. His plan is to create that enemy.
A colleague asks what will happen if something goes wrong; that leading scientists from around the world will instantly want to study this "alien from another planet". Dr. Gainer insists that the subject that has been chosen for this ruse is perfect in every way and once changed, will fool everyone, and then reveals the alien creature from the planet Theta that they will use as a genetic template for their creature. After the plan is revealed to the panel, a vote is taken. The names of all the members of the panel are placed in a container and Dr. Gainer pulls one out. It is the name of Allen Leighton.
At the research facility of the Advance Biological Studies Group, Dr. Gainer argues with Leighton about the extreme changes he will be going through. Leighton says he understands there is no turning back once the transformation begins but just before Gainer starts with the first injection, Allen's wife calls for him from the hallway. Leighton tells Dr. Gainer to proceed with the injection even with his wife still calling from outside the room. He then leaves the room and sees his wife waiting for him in the outer room.
As they walk along the street, window shopping, Yvette Leighton stops and looks at maternity clothing, prompting her husband to ask if there is a reason. She replies "no...not yet". For one of the reasons Leighton agreed to being the participant in the project was his wife's inability to have children because of a heart murmur. But after a checkup earlier that day, her doctor has given her a cleared bill of health to have children, which she says she's ready to have happen. The news shocks Allen and he asks his wife if something were to happen to him, how she would raise a child alone. She responds with a kiss meant to mean that this just isn't going to happen.
In Dr. Gainer's research facility, Leighton begins the transformation process. After he exits the pressurization chamber, Gainer asks him about the arrangements Allen must make for his "death". As Leighton rests, the mutations in his skin begin.
At home, in an intimate moment with his wife, she expresses her feelings of his upcoming "trip to the moon", which is his cover story of why he will soon disappear from sight, and eventually "die". Allen, for some reason, then tells his wife the plan, only in the form of a hypothesis, giving more information than he should. When Phil comes to pick Allen up to "go to the airport", Yvette suddenly changes her mind and decides she wants to accompany them to the airport also. Allen convinces her that she should stay home, causing a hint of suspicion in his wife.
At a secret manufacturing facility, Dr. Herschel takes Allen through the plan of launching him into space and then his re-entry back to earth, landing right in front of the United Nations. He also shows Allen his future spacecraft, made out of a new material never seen before on Earth to give the illusion that the craft is from outer space. As Leighton crawls into the cockpit, Gainer tells him to learn the control panel quickly as they need to return to the lab that night for more treatments. Allen then begins his training with an assistant.
After another session in the lab, Dr. Gainer asks Allen if he's had any reactions to the treatments. Leighton remarks that he can only sleep for periods of twenty minutes before waking in a sweat, and that the mental anguish of never seeing his wife again was terrible. Gainer reacts strongly, telling Allen to buck up to the mission, and Leighton tells him to cool off, that he's fine. Allen also asks if Gainer was ready to notify Yvette of his "demise". Phil tells him that he had just finished the telegram to show her since she was coming by the lab today. As Gainer steps out of the room, Yvette comes around the corner with her arms full of boxes after shopping, and asks if he had heard anything from Allen. The doctor says yes and hands her the telegram, explaining that Allen's plane went down somewhere over the Andes Mountains and hadn't been found. Yvette wonders out loud why she hadn't heard about it in the news. Gainer said that because it was a top secret mission, there was no news release, adding that if the plane is ever found that it would make the news then. Yvette doesn't believe the telegram. She says if Allen was dead, she'd know it. As she runs from the office, Allen emerges from the lab, having heard all through the door. He steps over to where his wife left her shopping boxes, to discover one from the maternity store they had looked into.
The procedures continue in the lab. As the doctors discuss what their next options are for the continued restructuring of Allen's body, he becomes faint. As Dr. Gainer examines him, he asks how Yvette is. Gainer tells him what he already knows, that she loves Allen very much, to which Leighton asks the doctor to take care of her for him. The doctors go back to their discussion on what the next surgical procedure will be when Allen starts to see things in a strange way and this leads to the beginning of a mental breakdown. Leighton, with his new super-human strength, starts to brutalize Gainer, knocking him across the room. As the doctors attempt to tie Allen down, he leaps around the lab, breaking up most of what he touches, flinging glass shards at the three researchers. Allen's breakdown continues, causing him to release the real alien from it's cage as he aims an x-ray machine toward the doctors. Two of the scientists sneak around the table to distract him as Dr. Gainer goes the other way to shut down the power to the lab. When Gainer kills the power and finds the x-ray machine in the dark, Allen is not there. Leighton has moved to the wall and is now trying to dial out on the phone. The doctors get to Allen just as Yvette answers the phone. She calls out repeatedly but there is no reply.
Days later, Allen awakes in the lab to Dr. Gainer explaining that he had suffered a schizophrenic episode and that they had needed to keep him sedated. As Allen asked if he had hurt anyone, Gainer said to not worry about it as they had discovered it had been caused by one of the hormonal extracts and had since been adjusted. Phil then informed him that they were continuing with the surgery.
Yvette, now fully suspicious after the phone call that something is up, arrives at the research facility and wants entrance to the lab. When she is stopped, she demands to see Dr. Gainer, who leaves surgery to tell her that she mustn't keep coming there. She tells the doctor that she came to get Allen's possessions and that she has a strange feeling that Allen is there. Gainer tells her to go home and think about her baby. She instead walks to Allen's office, which still has his name on the door. She walks into his office, touching the things on his desk and remembering their love for each other.
The surgery continues as they replace various organs. For a moment, it appears that Allen has died on the operating table. Yvette feels the death in Allen's office as she is going through his things. She exits the office just as the three doctors leave the lab with a shrouded body on a gurney headed for an elevator.
After a time of recovery, the three doctors gather to talk with Allen for the first time since the surgery. They affix a voice box to him so that he may communicate with them. Dr. Gainer asks him many questions about how he is and what he has been experiencing. Leighton tells of the various physical anomalies he has experienced and also of an initial deep depression that swept over him. Dr. Herschel then tells Allen of the arrangements to fly him to a remote island and launch him under the guise of being a weather satellite. All approvals have been received and no one else knows of the plan besides those on the panel. Once in orbit, his craft will be moved to the proper coordinates for re-entry and arrival in front of the United Nations building, where he is to walk directly into the General Assembly and deliver the news that many more are waiting to come behind him. Gainer then implores Allen to only use his laser weapon in an emergency. Allen then asks if he has any chance of succeeding in this mission. Gainer responds that many soldiers have gone into battle with much lower odds and ended in victory. Allen thanks him for that encouragement. The doctors then place Allen in a coffin in order to safely move him without being seen.
A few days later, the news of the unknown space craft on it's way to Earth is all over the news. As the spacecraft descends, it goes off target, causing the doctors to return to the lab in case Allen shows up there. The space vehicle lands in a remote area and is seen by three hunters with a dog. The dog takes off after it but soon returns and runs past the hunters at full speed. As they proceed toward where the craft landed, an alien hand with a weapon pokes through the reeds and discharges a beam, destroying the hunters' car. As the alien walks through the swamp, one of the hunters fires his rifle at him, hitting him. Yvette is walking downtown and suddenly feels the hit too. She hails a cab to take her back to the lab. As Allen stumbles through the marsh, Yvette arrives at the lab, finding the secure door open and walks in. Yvette examines the operating area of the lab when she hears a bizarre sounding noise and goes to investigate. She lifts a flap on the cage, only to drop it again quickly after viewing the small alien creature inside.
Allen, wounded by the hunter, stumbles down the stairs and into the lab, startling Yvette. As he sees her, he reaches out to embrace her but collapses because of the wound. The panel of scientists arrive to find Allen on the floor. He moves his hand to his head in a pattern that Yvette had once used to describe him and then points to her just before he expires. The realization hits Yvette that this is Allen and she screams out his name as she attempts to run to the alien creature on the floor. Dr. Gainer stops her, starting to explain what their plan was. She said that Allen had told her what the plan was only she hadn't known what he meant. Dr. Herschel said that it was Allen's choice when they had chosen the lots. Yvette then remembered what they had talked about, how a scarecrow could change everything. She then stoops down to hold Allen one last time as the lab assistants cover his body.