Colony

Scientist Seo Young-chul warns the police that he will launch a bio-terrorist attack at the Dongwoo-ri Building, then injects himself with the supposed vaccine to his mutated slime mold bioweapon. Meanwhile, Chains Bio is holding a presentation in the building, where CEO Kang Woo-chul demonstrates the slime mold's cell-regeneration capabilities. Kwon Se-jeong, an ostracised and unemployed professor, attends after being invited by the company, hoping to secure a job through her ex-husband Han Gyu-sung, who is preparing to emigrate overseas. Security guard Choi Hyun-seok is also at the building with his mobility-impaired older sister, Hyun-hee.

After the presentation, Young-chul confronts Woo-chul in a third-floor VIP room, accusing him of plagiarising his research before injecting him with the slime mold. Woo-chul rapidly mutates and infects nearby employees, triggering an outbreak. Counter-terrorism team leader Lee Bong-seok arrives with his squad, but the building is placed under lockdown, trapping them inside.

Se-jeong, Gyu-sung, Hyun-seok, Hyun-hee, Bong-seok, school bullying victim So-eun and several civilians become trapped in the shopping mall. During their escape attempt, Gyu-sung dies saving a delinquent female student. Se-jeong discovers that the infected possess advanced swarm intelligence, sharing sensory information through mycelium and rapidly evolving after each failed attack. When Bong-seok reveals that Young-chul caused the outbreak and possesses the vaccine, the group heads to the third floor to find him. Using perfume and antifreeze to evade the infected, they activate the fire sprinklers, temporarily paralysing the slime mold, and locate Young-chul before heading towards the 30th floor for evacuation. However, they discover that Young-chul can see through the infected's perspectives. He ambushes them and escapes, killing several survivors.

Meanwhile, Gyu-sung's wife, Gong Seol-hee, joins a research team investigating Young-chul's laboratory, established after his dismissal from Chains Bio. Infected monkeys escape and attack the researchers. When a SWAT officer kills monkey #1, the rest of the horde suddenly freezes.

The survivors move to the underground control room to find Young-chul. Hyun-hee stays behind, using CCTV monitors and a group chat to guide them safely. They find Young-chul, who deliberately allows Woo-chul to bite him, gaining complete control over the infected. During an argument, Bong-seok accidentally reveals the control room's location, allowing Young-chul's horde to overrun it. Hyun-hee deliberately gives false directions so Hyun-seok can escape and is killed. Devastated, Hyun-seok vows revenge and reveals the survivors' location over the intercom.

The survivors splinter. Bong-seok shoots a civilian to use as bait but is mauled to death alongside him. The delinquent student betrays So-eun by pushing her into the horde, but So-eun bites her to death after becoming infected. Hyun-seok confronts Young-chul, who summons the infected Hyun-hee. Broken by his sister's death, Hyun-seok stops resisting and is killed by her.

Young-chul confronts Se-jeong and reveals that he invited her to the event for revenge: she had exposed his father for embezzlement, leading to his father's suicide. A SWAT team breaches the area and detains Young-chul, but the officers are infected and become his guards. Young-chul escapes with them and unleashes the infected in the city centre, spreading the outbreak.

Guided by Seol-hee over the phone, Se-jeong disguises herself with a mycelium-covered jacket taken from a corpse and drives towards Young-chul. She throws the jacket over an infected person, causing hundreds of infected to form a collective, spinning ant mill. The resulting sensory overload crashes their shared system, reducing all the infected to a primitive state. Se-jeong hides beneath a bus while the disoriented horde attacks its advertisement, inadvertently pushing Young-chul into a burning car, where he dies. His death causes every infected person across the city to freeze.

With the only supposed vaccine gone, Se-jeong leaves the area and discusses with Seol-hee how to explain the infection to the public. As she walks away, however, one of the frozen infected suddenly moves.