NOTE: This spoiler was submitted by Jeremy

1983 – Up in the Orbita-4 spaceship, cosmonauts Konstantin Veshnyakov (Pyotr Fyodorov), and Kirill Averchenko (Aleksey Demidov) are preparing to return to Earth. As they begin their descent, their shuttle experiences turbulence. After things calm down, the men notice something crawling outside the shuttle that slowly makes its way inside. The shuttle then lands in Soviet Kazakhstan where they are found by other men. Averchenko is dead, with the back of his head blown out, while Konstantin is found alive but with his eyes completely blackened.

At a research institute in Moscow, Dr. Tatyana Yuryevna (Oksana Akinshina) is put on trial after her method in controlling a 17-year-old boy’s seizures was to hold him underwater for over a minute to cut off the oxygen supply. Although she saved him, other doctors and the boy’s mother think she went too far, but she knows she took a necessary risk. Afterward, she is met by Colonel Semiradov (Fyodor Bondarchuk), who was intrigued by her defense and requests that she join him at a facility in Kazakhstan to see what is wrong with Konstantin, as he is suffering from episodic amnesia.

Tatyana joins Semiradov at the facility where Konstantin is speaking to the research director, Yan Rigel (Anton Vasiliev), who is frustrated with a lack of progress in figuring out what is going on with Konstantin. Tatyana enters the room, and Konstantin instantly finds himself attracted to her. She speaks with him and sees that he keeps a nesting doll as a personal item. He explains what happened to him before Tatyana concludes that he is just acting out due to being in isolation in the facility. She is taken to her room to rest before hoping to depart the next morning.

Tatyana cannot fall asleep, so she goes for a run outside the facility. She bumps into an inmate who works outside the building, and he helps her up but won’t let go of her hand until another inmate separates them. Semiradov later goes to Tatyana’s room to see why they really brought her there. They observe Konstantin’s cell as a protective door is brought down. He convulses until a parasitic alien crawls out of his mouth. Tatyana stands just outside the door facing the creature, who lunges at the door. She is mortified by the discovery, and Semiradov explains that the parasite enters and exits Konstantin’s body without harming him, and he has no memory of any of it. What they want Tatyana to do is figure out a way to separate the parasite from the host without hurting Konstantin.

Tatyana stays at the facility to review more tests performed on Konstantin as she and the other researchers learn more about the parasite and how it appears to work. Tatyana also learns that Konstantin has a son that he left at an orphanage after his mother died so that he could continue with his work as a cosmonaut. She calls the orphanage to find out about the boy, who is seen to be a troublemaker. Tatyana then goes to Konstantin’s cell to ask him about the boy, but it only annoys him and makes him want to stop speaking to her. She then asks Semiradov to transfer Konstantin to the medical unit in the daytime so he can interact with civilians.

Konstantin’s transfer is made, and he is put in a bigger room with a TV where he watches news coverage of him and Averchenko returning home. He gets upset just as Tatyana enters and admits she wanted to see how he would fare under stress. Before she leaves the room, Konstantin admits that he only found out about his son a week before his initial flight and that the mother had died. He asks Tatyana what she really wants from him, but she cannot say.

That night, Tatyana observes the parasite near the cell as it makes its exit. She lays down on the other side from it, and it mimics her movements. It sees the nesting doll and responds to it. Tatyana puts on a protective suit and is allowed to enter the cell. She places the doll down, and the parasite wraps itself around the doll. When Tatyana tries to touch the parasite, she slips on its mucus, and the creature grabs her by the leg. The other scientists enter to pull Tatyana away, with her leg being injured from the attack. Afterward, Semiradov goes to Tatyana’s room, where she tells him that she thinks the parasite has now fully bonded with Konstantin since it reacted to the doll, which she thinks means something to Konstantin as a guilty reminder of his son. She suggests that Konstantin be moved to Moscow for proper research, but Semiradov admits that he has not even informed the higher-ups of Konstantin’s condition and that he lies to them about his condition to continue keeping him there.

Tatyana goes to Rigel for help in getting Konstantin out of the facility and over to Moscow so he can get the help he needs. That night, they quietly observe Semiradov’s men using the parasite for their own means. They witness the higher-ups bringing in an inmate into a cell where they wait for the parasite to exit Konstantin’s body and then tear the head off the inmate. Tatyana is almost discovered until the soldier that nearly finds her gets distracted. Before they continue with their plan, Rigel tries to tell Tatyana that they are on the verge of a big discovery with how the alien acts, but she argues that it will only be used to kill innocent people if left in Semiradov’s hands.

Tatyana spends more time with Konstantin as she plans to break him out. He admits to her that he does remember everything that the parasite experiences, but knows that if Semiradov knew, he would never let Konstantin go. He shows Tatyana that they have kept Averchenko’s body to continue observing him. She learns that he was suffering from Addison’s disease and that’s most likely why the parasite could not bond with him. Konstantin doesn’t want to end up like him, but he is also afraid to leave. When Tatyana talks to Semiradov again, he reaffirms that their goal is to find a way to control the parasite.

During another test in which Semiradov tries to have the parasite kill an inmate, Tatyana chooses to go in and sing “Million Roses”, which is a song that Konstantin is fond of. The parasite responds to it by remaining calm, indicating that it indeed does possess some of Konstantin’s memories. Rigel is impressed at the creature’s development, but the moment is ruined when it spots the inmate that it was supposed to feed on, and it proceeds to do just that. Semiradov then approaches a horrified Tatyana and learns that Konstantin still cannot fully control the parasite, so Semiradov will keep her there until that goal is achieved.

Later that evening, Tatyana and Rigel commence their breakout of Konstantin. Although he is reluctant to go with them at first, Tatyana convinces him and he leaves with her. Semiradov finds out and orders his team to go after them. He finds Rigel calling for help and executes him. As they are cornered by gunmen, Konstantin finally achieves control of the parasite and uses it to kill the soldiers. Tatyana and Konstantin get away, but Semiradov shoots the parasite, which Konstantin can feel.

As the day comes, Semiradov catches up to Tatyana and Konstantin with his men. He attempts to subdue Tatyana, but Konstantin controls the parasite to kill the soldiers and then tear off half of Semiradov’s head. Moments later, the parasite finally dies. Konstantin is okay, but as authorities are preparing to arrive, he has one final moment with Tatyana before he shoots himself dead to avoid any further containment. Tatyana is left devastated by the ordeal.

The film concludes with Tatyana going to the orphanage to adopt Konstantin’s son.

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In 1983, two Russian cosmonauts return to Earth with an alien parasite in tow, which takes control of surviving cosmonaut Konstantin Veshnyakov. Colonel Semiradov recruits controversial doctor Tatyana Yuryevna to determine what is going on with Konstantin since she is known for taking necessary risks.

Tatyana comes to sympathize with Konstantin after learning about his past and seeing that the parasite has a growing connection with its host. However, Semiradov wants to keep the parasite there to control it and use to kill inmates that he deems as less than human. Tatyana and the facility's research director, Yan Rigel, break Konstantin out, but Rigel is killed by Semiradov before he and his men go after Tatyana and Konstantin. When he and his men do catch up to them, Konstantin has managed to find a way to control the parasite to kill Semiradov and his men before the creature dies. Konstantin kills himself to prevent any further tests or to no longer keep him as a prisoner

Tatyana ends up adopting Konstantin's son from an orphanage after his death.