Colony

Scientist Seo Young-chul warns the police that he will launch a bio-terrorist attack at the Dongwoo-ri Building, subsequently injecting himself with the supposed vaccine to his mutated slime mold bioweapon. Meanwhile, Chains Bio is holding a presentation inside the building, where its CEO, Kang Woo-chul, showcases the cell regeneration capabilities of the slime mold. Kwon Se-jeong, an ostracized and unemployed professor, has received an invitation to the presentation, and arrives looking for a job opportunity within the company with the recommendation of her ex-husband Han Gyu-sung, who is preparing to emigrate overseas. At the same time, Choi Hyun-seok, a security guard, accompanies his mobility-impaired older sister, Hyun-hee, at the shopping mall inside the building.

Following the presentation, Young-chul confronts Woo-chul in a VIP room on the third floor, accusing him of plagiarizing his research before injecting the slime mold into Woo-chul's body. Woo-chul quickly mutates and infects nearby employees, resulting in an outbreak. Counter-terrorism team leader Lee Bong-seok arrives with his squad but ends up trapped as the building goes into lockdown.

Se-jeong, Gyu-sung, Hyun-seok, Hyun-hee, Bong-seok, school bullying victim So-eun, and several civilians become trapped in the building's shopping mall. During their attempt to escape, Gyu-sung loses his life while saving a delinquent female student. Se-jeong observes that the infected possess a high degree of swarm intelligence, allowing them to share sensory data in real time through mycelium and continuously evolve and learn upon every failed attack within a short period. Upon being informed by Bong-seok that Young-chul is behind the outbreak and has the vaccine, the group decides to head to the third floor to find him. The group uses perfume and antifreeze to sneak past, and triggers the fire sprinklers to disrupt the slime mold, temporarily paralyzing the infected. They manage to find Young-chul and head to the 30th floor for an evacuation. However, they discover that Young-chul can see through the infected's perspectives. Young-chul ambushes the group and escapes, causing the deaths of several survivors.

Meanwhile, Gyu-sung's current wife, Gong Seol-hee, joins a research group and investigates Young-chul's laboratory, built after his dismissal from Chains Bio. A horde of infected monkeys break free and attack the research group. One of the soldiers kills a monkey labeled #1, resulting in the rest of the monkey horde becoming frozen and unable to move.

The remaining survivors head to the underground control room to find Young-chul. Hyun-hee volunteers to stay behind, guiding them along a safe route using CCTV monitors and a group chat on their phones. They manage to find Young-chul, who intentionally allows himself to be bitten by Woo-chul, giving him total control over all of the infected. An argument breaks out between Bong-seok and Hyun-seok, the former planning to leave Hyun-hee behind, and he accidentally lets slip the location of the control room. Young-chul commands his horde to overrun the control room. To ensure Hyun-seok's safe escape, Hyun-hee intentionally gives false directions and is killed. Driven to mental breakdown, Hyun-seok vows to kill everyone in revenge and exposes the group's location through the intercom.

The group of survivors eventually splinters; Bong-seok shoots a citizen to use as bait, but ends up being mauled to death by the horde along with the citizen. The delinquent female student betrays So-eun and pushes her into the horde, only to be bitten to death by the infected So-eun. A vengeful Hyun-seok manages to intercept Young-chul, but Young-chul summons the infected Hyun-hee to stop him. His spirit completely shattered, Hyun-seok gives up all resistance and is stabbed to death by his own sister.

Young-chul confronts Se-jeong, revealing that he invited her to the event as revenge, as she was a whistleblower against his father for embezzlement, which led to his father's suicide. A special forces group breaches the area and forcefully detains Young-chul, but the whole group gets infected and turned into his personal guard. Leading the infected special forces, Young-chul escapes the building and unleashes the infected in the city center, triggering an outbreak on the streets.

Assisted by updates over the phone from Seol-hee, Se-jeong strips a jacket covered in mycelium from a corpse to disguise herself among the horde and drives a car to ram Young-chul. Se-jeong throws the mycelium jacket over the head of one of the infected, trapping hundreds of other infected in a collective, spinning ant mill. The massive influx of sensory data overloads and crashes the horde's collective system, reverting the intelligence of all infected to a primitive state. Se-jeong takes advantage of the chaos to slide under a bus to hide; the out-of-control horde shifts its focus to the bus's advertisement, pushing the nearby Young-chul into a burning car, where he burns to death. With the death of Young-chul, all infected across the city instantly freeze. With the only supposed vaccine gone, Se-jeong walks out, discussing with Seol-hee on how to explain the infection to the public. As she walks away, however, one of the frozen infected suddenly moves.