Two Russian cosmonauts on an orbital research trip in 1983, during the last hot era of the Cold War, witnessed something strange while returning to Earth: they saw something moving outside their ship. Only one of the cosmonauts, Konstantin, survives the spacecraft’s re-entry failure and crash in Kazakhstan. Colonel Semiradov, the commander in charge, recruits Dr. Tatyana Klimova, a young psychiatrist under examination for her unorthodox approaches, and transports him to an isolated military hospital. She is not told the true cause for Konstantin’s isolation, but she soon learns that the cosmonaut unknowingly took an extraterrestrial life-form back into his own body. The creature emerges in the middle of the night, when its host is sleeping. Colonel Semiradov explains that she has come to figure out how to separate Konstantin and the creature, as they appear to have developed a completely symbiotic relationship, with the creature living in the cosmonaut’s esophagus and benefiting from his nutrients, while Konstantin has made an impossible recovery from his crash-landing injuries.
Tatyana confronts Konstantin by informing him that she knows he has a child who he has abandoned in an orphanage, implying that he is not a national hero but a coward who is unconcerned about his son, in order to put him under duress. She discovers that the creature is affected by hormone levels and that one of the videos she has been shown has been altered as a result of this, which leads her to discover a terrifying truth: the creature does not feed on Konstantin’s food, but rather on live humans, and Semiradov has been feeding it prisoners. The alien appears to its victims in order to increase their anxiety, which causes the human brain to release cortisol, which the alien feeds on. She approaches Semiradov, disgusted and terrified, only to be told that the real reason they’ve come is to find a way to turn the extraterrestrial into a weapon they can control. He asks if she will cooperate, and she appears to say yes.
Later, because she has formed a sympathetic relationship with Konstantin, she takes him for a run around the facility, where she informs him that he has a parasitic organism inside him and that the military has plans for him. They agree to meet later, when she shows him the bodies of the people the creature killed, as well as the still alive astronaut who was in the spacecraft with Konstantin: he was not infected because he had cancer, which had gone undetected by pre-mission tests but which the creature must have sensed. Konstantin was well aware of his unwanted visitor and what the thing does when it is not tied to him, it is revealed. Tatyana is taken aback by this news, and Konstantin informs her that in order to care for his son, he will go to whatever length to get out of this situation.
Tatyana finally decides she wants to aid Konstantin and plan an escape with the help of another doctor inside the facility. She hands Konstantin a syringe containing medications that will mimic Addison’s illness, the same disease that the other cosmonaut suffered from. She reasoned that this would technically push the thing out, and that it would then die within an hour of being outside its host.
Semiradov ambushes them as they flee and dispatches a response force to pursue them. Konstantin begs Tatyana for the syringe since she is hurt. The alien appears after injecting himself and kills the team. Tatyana and Konstantin flee, but Konstantin collapses before they can get very far. When Semiradov catches up with them, Tatyana sees that Konstantin can’t exist without the alien because they’ve become entirely symbiotic, and she surrenders. The Colonel has brought the now severely injured creature to Konstantin in order to force it inside; instead, the latter has the creature kill Semiradov and his companions, freeing Tatyana and himself. However, when the authorities approach, having been informed to Semiradov’s monster experimenting, a revived Konstantin shoots himself in the head in order to kill the beast, as well as himself.
Tatyana later adopts Konstantin’s son, and it is revealed that she was raised in an orphanage as a child.