In 1984, the story revolves around Anna Fedorova, a Soviet epidemiologist who makes a controversial decision to bypass pre-human vaccine testing to meet state deadlines. Her research associate, Dr. Zotoff, chooses to test the vaccine on himself but tragically dies as a result. Faced with immense shame, Anna considers resigning from her position, yet her superior, GRU Colonel Morozov, persuades her to continue her vital work.
On New Year's Eve, Anna celebrates her achievement with family and friends, but her celebration is interrupted when Morozov contacts her regarding a clandestine mission at the Kola borehole in Murmansk. Contrary to the public belief that it is merely a dig site, it is actually home to a secret laboratory buried deep in the permafrost. Mysterious sounds have been recorded from beneath the surface, coinciding with the disappearance of twenty people from the site. The objective of their mission becomes critical: they must obtain samples from a suspected infectious outbreak hidden by the facility's head, Dr. Grigoriev.
When Anna, along with Morozov and soldiers led by Major Mikheev, arrives at this ice-encased facility, they encounter a lab technician who suddenly approaches them. In a shocking turn of events, the on-site soldiers attempt to shoot him multiple times, but he appears unaffected and ultimately takes his own life using a grenade. In the chaos, Anna is forced to collect samples from the body, which begin to decay rapidly in the freezing conditions. As Anna investigates the evacuated staff, one warns her ominously of deception and cautions that the lower levels of the facility resemble Hell.
The team soon meets Deputy Head of Research Peter Kuznetsov, who openly denounces Grigoriev and insists on guarantees for the rescue of remaining personnel trapped in the lower sections. Grigoriev, having sealed all other accesses and altered the elevator codes, compels them to accompany him. As they descend into the facility, Grigoriev dangerously depressurizes the elevator cabin, thereby incapacitating the team, before escaping into the "Resort" residential area with a key that is crucial for their mission.
Inside the Resort, the team discovers engineer Nikolay and Dr. Kira, who were left behind during the evacuation. Upon testing them in Kira's lab, Anna finds no trace of infection. Their encounter with Grigoriev proves tense as he begs them to exit via the shaft while stubbornly refusing to relinquish the key. After an exchange of violence, he manages to flee to the "Sahara" research module. Following him proves perilous due to the extreme heat, compelling team member Egorov and his men to don protective gear. Unexpectedly, Olga, a lab assistant from Sahara, emerges from the overheated shaft, and is transported to the lab for evaluation.
As Olga exhibits signs of fever while feeling cold, a disturbing fungal growth emerges on her body, prompting Grigoriev to announce over the intercom that he must seal off the facility and proceeds to detonate the pressure pump, initiating a collapse of the Resort within an hour. Returning to the lab, Anna and Mikheev are horrified to find the walls covered with a black mold. A horrifyingly mutated Olga, now half-melted into the floor yet alive, releases spores that infect Mikheev. In a moment of desperation, Anna slips on a gas mask, narrowly defending herself against a mutated Kira. Recognizing that the mold thrives in warmth and dies in cold, Anna uses a fire extinguisher to decontaminate herself.
As Anna explains to Morozov that the permafrost is preventing the mold from expanding, he reveals the shocking truth: he cannot call for rescue as he is determined to secure a biological weapon for the state, intending to bring the increasingly infected Mikheev to the surface. Meanwhile, Egorov frenetically reports an imminent attack, prompting the last three soldiers to attempt a rescue despite his warnings. However, only two return, having encountered something enormous and impervious to their gunfire. The chaos culminates in one soldier, missing an arm and pursued by something unseen, taking his own life.
The remaining soldiers, along with Nikolay, triumphantly reach the frozen Sahara, where they discover a bizarre wall composed of the missing infected scientists who have melted together but have been preserved by the cold. Grigoriev has left with the key, sabotaging the heat suits in the process. Morozov volunteers to retrieve the key but does not come back. Anna manages to reclaim the key from his corpse and discovers the epicenter of the outbreak—a colossal, pulsating fungal super-colony. In a tense moment, Peter seizes the key at gunpoint, injuring Nikolay as he yearns for recognition as the one to officially unveil the discovery of the mold.
In a chaotic encounter, the survivors return to the Resort only to be accosted by a horrifying amalgamation of organic matter formed from fused bodies, which violently absorbs Peter. In a desperate act, Anna hides in a freezer to elude detection and escapes with Nikolay, only to find a mutated Mikheev fused into the elevator floor. His plea to Anna is heart-wrenching: he begs her not to allow the infection to spread. Nikolay's eagerness to flee ignites a bitter struggle for control of the elevator, eventually leading Mikheev to free himself partially and fatally stab Nikolay in a moment of sacrifice for Anna's sake. Just before his passing, he confesses his love for Anna, declaring himself Yugoslavian, as they share a poignant kiss.
Facing her own infection, Anna drops the elevator down, but before succumbing to despair, she is snatched away by a hazmat team. Once outside in the biting cold, she seizes a grenade in a final, desperate attempt to eliminate the mold along with herself. Just as a soldier stealthily approaches her with a sedative, the screen fades to black as she readies the grenade, leaving the audience in suspense over her fate.
