The planet Ebon has launched an unprovoked attack against Earth. In response, Earth has sent troops to Ebon to engage the enemy. None have returned and there has been no word from those who have gone before.
Colonel Luke Stone, United States Astro Force, is leading the next mission to Ebon, preparing his troops for what they may find, when a piercing high-pitched tone begins, causing extreme pain to the men in the ship. The ship comes under attack by an unknown force, and the next thing the men remember is marching in single file on the planet's surface.
As the men stop, a voice instructs the soldiers to turn toward it and one at a time, say their name. Colonel Stone, Lt. Krug, Capt. Brookman, Maj. Jong, Lt Willowmore, and Pvt. Dix, all speak their name and rank. The voice informs the men that they are now prisoners of war, to cooperate and not try to escape, and that their space ship has been destroyed.
The prisoners from Earth are commanded to proceed to compound 0-3 but as they march, Dix begins to break, believing that aliens are affecting him in an unseen way. Colonel Stone runs toward him but is commanded to halt by a Ebonite soldier, who then points a device toward Dix that renders his voice inactive. Stone and Willomore pull Dix to safety and ask if he is alright but Dix is unable to make a sound.
As the men look back to the alien, a box floats into range, which, through speakers, welcomes the men to compound Zero Three. The voice is that of the interrogator whom the men met when they first got to the planet. The voice asks if they have appointed a spokesman for the group and Colonel Stone steps forward, making demands for his troop, only to be stopped by the interrogator, acknowledging that he is aware of their needs after dealing with the previous group of POWs, whom he says are cooperating and contented. He promises that if the soldiers will also cooperate, then their needs will be provided. Stone strongly implores medical attention for Pvt. Dix but the voice ignores the demand, telling Stone that each man will be interviewed, starting with Dix.
When the voice instructs one of his troops to escort Dix to his quarters, Dix goes berserk with fear, as Stone tries to calm his man, promising he won't be hurt. Stone turns to the alien escort and demands that Dix not be hurt. Lt. Willowmore approaches the alien from behind and is hit with a beam that renders him blind. The alien takes Dix away as Stone tends to his man.
Pvt. Dix is delivered to an interrogation room and begins to look around as the voice of the interrogator instructs him to sit down. He tells Dix that his inability to speak may only be in his head and tells another alien to bring the frightened man water, which he is then told to drink. The interrogator now tells Dix to talk and the space soldier tries to speak but can only mouth his name, rank, and serial number, to which the interrogator says is not good enough and only useful on Earth for his relatives to collect insurance. Suddenly, behind Dix, his mother appears and says, "I want my son, not his insurance." Shocked by the image, Dix stands up and runs toward his mother and hits a wall, realizing that it was a projection only.
As Dix sits on the floor, feeling the wall where the image of his mother had been, a voice is heard outside the door demanding to see the prisoner. A "Doctor Whorf" appears and and tells Dix he's a friend as he helps the crewman to his feet. He explains that the Ebonites can control all five senses and that is why Dix can't speak and that they can restore his voice whenever they want to. The doctor tells Dix that he tested him during his induction to the military. The psychiatrist tells the young soldier that he remembered the result of his examination which revealed in such a young man, a tendency toward self destruction and that he is destined to destroy himself. As an example, he postulates that as he's been trained, he'll refuse to cooperate with his captors, as Dix shakes his head in the affirmative. Whorf says that Dix will be tortured for his lack of cooperation and die because of it. He continues to push Dix to do what the Ebonites require as Dix starts to figure out what is going on and the doctor is told to leave.
The interrogator asks if he's ready to speak and Dix mouths his name, rank and serial number again, to which the alien says that maybe he'd feel like talking if there was someone there he really wanted to talk to. The image of Dix's mother appears again, telling him that it's okay to speak with her. Dix opens his mouth and says, "Mom" out loud. He stands and approaches his mother, able to put his arms around her as she holds him tight and laughs. Dix weeps as she hugs him. She encourages her son to talk, that he can just whisper what they want to know to her. Dix whispers in his mother's ear when suddenly, the interrogator tells him that he may go. Dix, with his arms still in the shape of a hug, then realizes that he's been hugging and speaking to an illusion.
The men hear Dix yelling that he can talk again as he runs back to the compound. He tells them about whispering to his mother, then quickly adds that he didn't say anything. Major Jong responds that he had been under a hallucinogenic drug as the voice calls out for Lt. Krug to proceed to the interrogation room. Willowmore, hearing that Dix regained his voice, yells out to be next so he can get his eyesight back. Krug is again commanded to respond. He begins to explain that he fears a truth will come out under interrogation that he is not a pure German, but that his grandfather was Jewish. Krug then reluctantly goes with the guard. On his way, he "sees" his governess who accuses him of crying and of not being "pure". As he turns, he sees Dr. Whorf who tells him to get it over with, and then tells him he knows Krug turned in his Jewish grandfather to the Nazis. As he looks back to Whorf, he now sees the image of his grandfather, in Hebrew clothing, telling Krug he was forgiven. Krug collapses from the shame.
At the compound, Dix assures his fellow prisoners that he won't talk. Jong asks him if he'd heard about how they extracted information during the Korean war, to which Dix shakes his head no. Jong continues that since the Ebonites control all the senses, that they should have come up with a totally different form of execution. Dix runs away from this kind of talk as Jong predicts that whatever form of death it is, it will be an adventure. Meanwhile, Krug succumbs to a device that is attached to his chest.
Stone suddenly wakes up in the chamber to the voice of the interrogator telling him that he may go. As a guard leads him away, the interrogator reminds him to inform his men of the fate of Lt. Krug. Stone tells his men that Krug is dead, having gone mad during his torture, that madness is not allowed on Ebon. Captain Brookman enquirers about Stone's interview and Stone says it was just routine questions about the next ship's arrival, it's flight path, and that he fell asleep during questioning. Willowmore tells Colonel Stone that he is thirsty and Stone helps him drink from the fountain, telling him that he might as well drink the drugged water there since he'll have to drink it in the chamber. Just then, Willowmore is called for questioning and is lead away saying he'd "see" everyone when he returned. Stone tells him to not be seduced by a promise, to which Willowmore responds, "no sir, promises just roll off my back."
Captain Brookman, still bothered by Stone's confession that he fell asleep during questioning, confronts the colonel as he sits down. Stone warns Brookman about his tone of voice and that as they continue, things will only become worse as they become more suspicious of each other, which is interrupted by Major Jong's laughter. He is confident after witnessing his fellow Earthling's human emotions fully in play, guaranteeing their survival. Brookman questions Jong's motive as Willowmore returns to the compound. He explains what happened to him, that they asked him if he wanted to see and if he would look at Krug's body. Wanting to see again so strongly, he agreed, only to be repulsed at the sight of Krug with his heart missing.
Back at the chamber, the interrogator looks on as what appears to be a high ranking officer walks into the room where Captain Brookman is sitting. Brookman responds to the General's greeting by asking if the illusion will stop if they shake hands. The General asks if Brookman thinks he's not real and then begins a long conversation about the evils of war, the waste of human life, and that he needs to cooperate with the aliens or their could be consequences. After the General is told to leave, Brookman gives his name, rank, and serial number which irks the interrogator, who informs the Captain that time is running out for the members of the next Earth ship approaching the planet. The aliens have not been able to break the scrambled signals the ship uses and they must know this information to intercept. Brookman once again gives only name, rank, and serial number and is lead away as two officers look at slides of the captured crew members, discussing who they think will serve their purpose.
Brookman is back in the compound, telling the others that he thought the chamber was being readied for him, only it was Jong that was taken there and tortured. Just then, a plate of block like material is placed in front of the men as the oracle announces that this is safe edibles, and that there is no longer any need to guard them. Brookman and Stone discuss why they were now given "honor and respect" and the conclusion was that Jong talked during the torture. The discussion continues as the others begin to suspect each other, leading to a physical confrontation between Dix and Willowmore which is broken up by Stone. Jong appears back in the compound and Dix makes a remark about getting food because of him. Jong says nothing and walks to get some of the food when Dix yells "traitor" and spits on Jong, prompting Stone to restrain Dix. Jong says they never gave him a chance to be a traitor and simply pulverized his right arm. The men just look in shock at the limp arm as the sentinel sounds again, announcing the arrival of the interrogator, who congratulates the troop for selecting their traitor and cooperating. Stone responds that there had been no traitor among them, to which the alien reads out all the information gathered concerning the approaching ship.
After the interrogator leaves, the men begin to try and determine who betrayed them. Each of them knew only bits of the information the alien had, but only Jong had all the information in total. Stone takes a handkerchief and begins to rip it into strips. When asked why, Stone says that one of them must kill Jong and when asked why, says that there can be no leniency for a traitor. As Stone tells Brookman to chose a strip, the approaching ship is shot out of the sky over their heads. After seeing their compatriots killed, they all begin to choose strips. Brookman pulls the short straw but can not bring himself to kill Jong which leads to a loud discussion on the morality of what they are asking. Brookman wants to wait until they get back to earth for a trial when Jong asks Dix what he whispered to his mother. At this, Dix sees his mother and begins to cry on her, wanting to be reassured that he didn't tell her anything. The other men then realize that it had to be Dix who gave up the information. Dix, in a hysteria, runs out of the compound only to be confronted by a guard and his weapon. Stone catches up to Dix, but it's too late. He's suffered a complete mental breakdown.
As two officers look over star charts, the interrogator confronts them with what has happened so far, one man dead, one insane, one with a mangled arm, and informs the officers that this must stop now and the men must be informed of what is going on. The officers object as Colonel Stone walks in. He hears that this is a game and wants to know the truth. The alien informs Stone that they are not at war, that the missle attack on Earth was a terrible mistake and that Ebon had promised to do anything they could to make up for it. The officers then inform Stone that they were using this opportunity to study their men in a war game, to see the psychological affect that an interplanetary war would have on space soldiers to help in training so they could avoid the informing that took place in the Korean war. The superiors tell Stone that they must complete the phase that is going on with the men having to decide if they are going to kill Jong. Stone refuses the order and is kept from leaving by the officers as the alien steps away into the shadows.
In the compound, the men discuss their situation, still thinking they are captives of an alien race and are being killed one at a time. As Colonel Stone runs from one of the officers, the interrogator steps into the compound to tell the men, only they jump him and attempt to kill the alien when Stone and the officer approach. Stone orders his men to stand down, but Jong keep his arm around the alien's neck. The officer draws a gun and tells Jong that he'll kill him if he doesn't let go but from behind, Willowmore knocks the gun from his hand. Brookman recovers the gun as Stone orders him not to shoot. But shoot he does, killing the human officer. Brookman said he thought he was an alien as the other officer arrives and the alien is let go, leaving the men to stare at their dead superior, wondering what it all means. They will eventually be told that they were part of military games played on an alien planet to test how men in space will handle the stresses of war in space. It will do little to console these men.