An urgent confrontation between science, ambition, and survival unfolds inside a single towering building. Scientist Seo Young-chul [Koo Kyo-hwan] warns the police that a bio-terror attack will be launched at the Dongwoo-ri Building, then self-administers a vaccine-like injection to a mutated slime mold bioweapon, setting off a catastrophic chain of events. In the same building, Chains Bio stages a high-profile presentation where its CEO unveils the slime mold’s cell regeneration capabilities, drawing attention from researchers, investors, and workers who are eager to witness the breakthrough up close.
Among the attendees is Kwon Se-jeong [Jun Ji-hyun], an ostracized and unemployed professor who has been invited to the event, hoping for a new opportunity with the company. Her invitation comes with the quiet support of her ex-husband Han Gyu Seong [Go Soo], who is preparing to emigrate overseas and sees a potential chance to move forward through this chance encounter. At the same time, security guard Choi Hyun-seok [Ji Chang-wook] accompanies his mobility-impaired older sister, Hyun-hee, through the mall-like spaces inside the building, trying to keep her safe as tensions rise.
What begins as a controlled demonstration quickly spirals. After the presentation, Young-chul confronts Woo-chul in a VIP room on the third floor, accusing him of plagiarizing the research. The confrontation ends with a brutal act: Young-chul injects the slime mold into Woo-chul’s body. The mutated organism spreads rapidly, infecting nearby employees and turning the building into a controlled environment for a spreading outbreak. A counter-terrorism unit led by Lee Bong-seok arrives with his squad, only to find themselves trapped as the facility locks down and the danger intensifies.
As the shopping mall within the building fills with terrified people—Se-jeong, Gyu-sung, Hyun-seok, Hyun-hee, Bong-seok, a school bullying victim named So-eun, and other civilians—the situation becomes a fight for survival. Gyu-sung sacrifices himself to save a delinquent student, a moment that underscores the human cost beneath the chaos. Se-jeong notes a disturbing property of the infected: they exhibit a high degree of swarm intelligence, able to share sensory data through the network of mycelium and learn from each attack, evolving at a frightening pace.
With the clock ticking, Bong-seok reveals that Young-chul is behind the outbreak and possesses the vaccine. The survivors make a dangerous bid to reach him on the building’s upper floors, using perfume and antifreeze to mask their movement and triggering fire sprinklers to disrupt the slime mold long enough to buy time. They push toward the 30th floor in search of an evacuation route, only to discover that Young-chul has a devastating new advantage—he can see through the infected’s perspectives.
Young-chul ambushes the group and escapes, causing several more deaths among the survivors. Parallel strands unfold: Gyu-sung’s wife Gong Seol-hee joins a research group investigating Young-chul’s laboratory, a facility built after his dismissal from Chains Bio. A wave of infected monkeys breaks loose, forcing a swat of officers to respond; a pivotal moment occurs when an officer kills one monkey labeled #1, stilling the rest for a moment as they freeze.
The remaining survivors navigate toward the underground control room, where Hyun-hee volunteers to stay behind, guiding the others via CCTV monitors and a mobile chat to stay on a safe path. They eventually locate Young-chul, who deliberately allows himself to be bitten by Woo-chul, gaining total control over the infected. An intense argument erupts between Bong-seok and Hyun-seok about what to do next, and the revelation of the control room’s location becomes a dangerous liability.
The group splits under mounting pressure. Bong-seok shoots a civilian to use as bait, but both he and the civilian are torn apart by the horde. The delinquent student betrays So-eun, pushing her into the crowd, only to be bitten by the infected So-eun in turn. A vengeful Hyun-seok closes in on Young-chul, but Young-chul summons Hyun-hee’s infected presence to halt him. Driven to a breaking point, Hyun-seok charges forward, and in a heartbreaking turn is fatally stabbed by his own sister.
In a final face-off, Young-chul confronts Se-jeong and reveals his motive: he invited her to the event as revenge because she had been a whistleblower against his father for embezzlement, a revelation that ties personal vendetta to a city-wide catastrophe. A SWAT breach interrupts, and Young-chul is briefly detained, but the group they thought they’d saved becomes his elite guard, infected with his control. He leads the infected SWAT units out into the city, unleashing chaos on the streets.
Se-jeong, with Seol-hee’s help—who provides crucial updates from the outside—uses a jacket soaked in mycelium to disguise herself within the horde and drives a car to ram Young-chul. She then casts the mycelium jacket over the head of one infected, triggering a cascade that overwhelms the swarm’s collective brain. The sensory flood crashes the hive mind, returning the infected to a primitive state, and Se-jeong seizes the moment to hide under a bus as the horde falters. The distraction pushes Young-chul into a burning car, where he dies, and, for a moment, the city breathes as the infected freeze in their tracks.
Yet the ending carries a chisel-like note of ambiguity. With Young-chul dead and the virus seemingly contained by the collapse of the hive mind, Se-jeong contemplates how to explain the outbreak to the public. As she walks away, a single infected figure suddenly stirs, leaving a chilling implication that the threat may not be fully vanquished and that the city’s ordeal could reawaken in an unexpected moment.
