As the Orbita-4 spacecraft races back towards Soviet Kazakhstan, cosmonauts Konstantin Veshnyakov and Kirill Averchenko find themselves on the brink of re-entry. However, their atmospheric descent becomes chaotic when an uninvited presence infiltrates their shuttle. Initially anxious, the situation rapidly spirals into horror when Averchenko's lifeless body is uncovered, his skull brutally shattered. At the same time, Konstantin lies still, his eyes transforming into dark voids.
In Moscow, Dr. Tatyana Yuryevna (Oksana Akinshina) finds her professional integrity under scrutiny. Her unorthodox approach has sparked admiration, but also skepticism among her colleagues, particularly regarding her decision to submerge a 17-year-old boy underwater for over a minute. Despite successfully saving the boy's life, she faces a tribunal that draws the attention of Colonel Semiradov. He invites Tatyana to Kazakhstan, where her expertise is needed to unravel the enigma surrounding Konstantin's disturbing condition—one that has led to episodes of amnesia.
Upon her arrival, Tatyana meets Yan Rigel (Anton Vasilev), the beleaguered research director struggling to make sense of Konstantin's erratic behavior. As Tatyana interacts with Konstantin, she observes a sudden, inexplicable attachment forming between them, symbolized by a nesting doll that resonates with Konstantin's emotional turmoil. Initially, she believes his actions stem from the psychological trauma of isolation. However, as the night of her arrival unfolds, she suspects that the situation is much more intricate than it appears.
Driven by insomnia, Tatyana takes a nighttime jog around the facility, where she encounters an inmate who assists her but refuses to let go of her hand until interrupted by another prisoner. Later, Semiradov visits her, digging into her reasons for being there. Their conversation is interrupted by a shocking sight; a protective door crashes, revealing an alien parasite that emerges from Konstantin's mouth. Tatyana is left in stunned horror as Semiradov explains that this alien creature oscillates in and out of Konstantin’s body without harm, introducing a new layer of complexity to Tatyana's mission to liberate him from its grip.
Delving deeper into her research, Tatyana learns more about Konstantin's haunted past, which includes leaving his son at an orphanage after his wife’s death in order to focus on his space career. When she inquires about the boy, she finds he is mischievous and difficult to manage. This prompts another visit to Konstantin, but her questions only exasperate him, leading him to withdraw.
To facilitate a better understanding of Konstantin's psyche, Tatyana convinces Semiradov to move the cosmonaut to a medical unit where he can mingle with civilians. In the new room, Konstantin sees a news report celebrating his return, revealing the weight of trauma he carries. Their bond deepens as he finally confides about the son he only recently learned of, casting a poignant shadow on his situation.
As the tension heightens, Tatyana becomes increasingly fixated on the alien parasite, observing its mimicry of her own actions one night. In an attempt to communicate, she presents the nesting doll to the creature, prompting hazardous consequences when she slips in its mucus and becomes temporarily ensnared. Her colleagues come to her rescue, but the incident leaves her shaken and alert to the creature's potential threat.
Recovering, Tatyana realizes the parasite has bonded with Konstantin. She urges Semiradov to relocate Konstantin to Moscow for proper research, yet Semiradov's true motives emerge as he confesses to hiding Konstantin’s condition from their higher-ups, indicating a sinister agenda. Understanding the gravity of the situation, Tatyana knows she must seek assistance outside the oppressive environment.
As plans unfold, Tatyana and Yan engage in clandestine surveillance, witnessing Semiradov's team exploiting the parasite to achieve deadly results. While Rigel cautions her about the scientific breakthroughs potentially exploding from this situation, Tatyana remains steadfast in her belief that such discoveries could lead to horrific outcomes if left unchecked.
Their desperate mission intensifies when Konstantin shares chilling details of his experiences with the alien entity, revealing the scars left behind by Semiradov’s unscrupulous pursuits. Determined, Tatyana confronts Semiradov one last time, pleading for him to recognize the humanity within Konstantin’s plight. Her resolve leads her to use music to unlock Konstantin’s memories, but Semiradov's harsh methods serve as a reminder of the precarious position they are in.
The ultimate confrontation is fraught with tension, culminating in a heart-racing struggle for freedom as they push back against Semiradov's oppressive regime. Rigel pays the price for their escape, falling prey to Semiradov's malignant tactics, while Konstantin's link to the parasite breeds immeasurable power, allowing them to evade capture.
Through thick chaos and relentless pursuit, Konstantin grapples with the truth of his connection to the parasite, facing Semiradov with an unwavering determination to protect Tatyana. In the climax, he makes a sacrificial choice, showcasing his profound love and unyielding will against the system that has tormented them.
In the wake of this tumultuous finale, Tatyana is left to confront the weight of their shared trauma. With a heart full of conflict, she decides to adopt Konstantin's son from the orphanage, a beacon of hope amidst the darkness that shadows their experience. The promise of a new beginning emerges, standing in stark contrast to the anguish they have endured.
