Dr. Paul Thorson is sent to an Alaskan research base to investigate the staff's loss of contact with the outside world. The researchers are working on Project Gemini, a secret research project that Thorson is supposed to put back on track. The helicopter drops him off and he enters the base. He finds a paranoid corporal, Gordon, firing at a door leading to a secure lab. Thorson tries to calm Gordon down, but the corporal says that "he" has locked himself in the lab and is preparing to activate Gemini, destroying them all. Thorson tries to explain that Gemini isn't a destructive device but Gordon insists that "Chandler" is the one responsible. Thorson hasn't heard of anyone called Chandler and can't find the name in the staff log. Gordon claims that Chandler created Gemini and then went nuts, driving all but three station personnel out into the snow at gunpoint to freeze to death.
Thorson offers to open the combination using his security code but it doesn't respond. Gordon begins to suspect Thorson, who hastily explains that he's the only one who can shut Gemini down so Gordon needs him alive. Satisfied for the moment, Gordon says that the other two survivors are Skyles and Morgan.
Thorson goes to find Dr. Morgan, who is working over equations in a lab. She insists that her equations are vital and Gemini won't work until she completes them. Thorson instinctively solves the equation for her but realizes that Gemini would unleash massive uncontrollable amounts of damage, and that it's only use is for a weapon. Morgan continues to obsess about the formula and says that Chandler is talking to her. She talks back to Chandler while Thorson tries to get the secure lab's combination. She finally tells him to talk to Skyles, who is in the communication room.
Thorson finds Skyles playing video games. The commander gives him a gun and tiredly says that he figures Thorson is there to kill him. He has resigned himself to the fact that everyone will blame him for Chandler killing the others, and insists Chandler is talking to him. Thorson says that Chandler is a shared delusion but then discovers that someone has wrecked the communication equipment. Suddenly he hears Chandler talking to him, saying that he has to leave. Thorson goes back to the secure lab door while Chandler continues to talk to him, saying that the other researchers realized what their work on Gemini meant and went insane.
At the secure lab door, Thorson insists it's some kind of a trick but can't find any hidden loudspeakers. He takes an ax and tries to break down the lab door without success. Gordon insists that if they don't get the door open, they're all going to die. Thorson brings Morgan and Skyles to the door while Chandler warns him that it's hopeless. Thorson refuses to believe it and tries to rally the others. As Chandler continues to taunt them, Thorson realizes that each person is hearing something different from the man. Believing the entire thing is a delusion, Thorson tries the combination again and Gordon attacks him. The two men struggle over a pistol and Thorson ends up accidentally shooting Gordon. He looks on in horror as the corporal disappears into thin air.
Questioning his own reality, Thorson shoots first Morgan and then Skyles. They disappear as well, and the secure lab door opens on its own. He goes inside and finds Chandler, an older man, who explains that Thorson created Gemini. When Thorson realized how destructive it was, he went insane. Chandler explains that Gordon, Morgan, and Skyles are parts of Thorson's own personality: Gordon his paranoia, Morgan his obsessive nature, and Skyles his depression. Chandler says that the only way that Thorson can assure the safety of the world is to kill himself. Thorson refuses to believe it and shoots Chandler. As Chandler falls to the floor, he transforms into Thorson, who can only look at "himself" in horror as he dies.
Admiral Munro sends a team of soldiers to secure the base. They find Thorson dead and the computer files wiped. The team commander determines that Thorson killed everyone else and finally killed himself. Munro tells his men to bring the research back and they'll find someone else to complete the project.