Deranged

Jae-hyuk, a former biochemistry professor, loses his life savings and job after a bad stock investment advised by his younger brother, Jae-pil. With mounting pressure, he shifts into pharmaceutical sales to support his family. The personal crisis sets the stage for his involvement in a national medical emergency.

Dead bodies appear in the Han River, shocking the public. Authorities identify the culprit as Yeongasi, a brain-controlling parasite that mutates to infect humans. Infected individuals exhibit hunger and thirst as the worms mature, driving some to jump into the river to release the parasites.

Across the country, bodies are found with signs of malnutrition as more victims fall ill. Parasites escape from the bodies when they are in water, revealing their mutated ability to infect humans. Officials struggle to explain the strange deaths as the public grows increasingly fearful.

Public alarm spikes as hospitals fill with patients and pharmacies stockpile anthelmintics. Early cures prove ineffective, and attempts to treat infected individuals cause more pain and suffering. The crisis deepens as more people jump into rivers in search of relief.

A patient reports being cured by Windazole, turning the drug into a beacon of hope. Demand explodes while supplies run dry, and officials urge production to meet the surge. ChoA Pharmaceuticals is pressured to increase output, facing scrutiny over its manufacturing after the company's sale.

Following an acquisition by an investment company, ChoA struggles with production delays and mounting stock concerns. The government seeks to force disclosure of the drug's composition, but the CEO cites stockholders' refusal and legal limits. The nation spirals as people clamour for any hint of a cure.

Jae-pil digs into ChoA's past and locates a former researcher willing to talk. The confessor explains that the company’s team pursued the parasite to aid brain research but instead orchestrated a stock-manipulation scheme. The scheme released the parasite to boost profits during the tourist season.

The investigators corner the storage facility to seize remaining doses, but a lockout and fire trap them inside. They survive the blaze, yet the insider who aided them is killed. The researcher who had guided them is revealed to have been the project head and a close ally of the CEO.

The murdered insider is revealed to be the head of the team who kept the rest of the drugs, and he was a friend of the CEO. Most other researchers have already fled overseas, leaving the CEO to push the plan to the end. The revelation confirms the conspiracy existed from the start.

The Prime Minister contemplates a buyout as a means to access the drug’s recipe, but investigators reveal the entire scam first. The government shuns the buyout, and the CEO is arrested in a swift crackdown. The moment marks the turning point from corporate greed to public-interest justice.

Leveraging his chemistry knowledge, Jae-hyuk realizes he can recreate a drug with the same active compound as Windazole to treat the infected. He shares the idea with allies and begins mobilizing labs to produce a generic solution. The plan shifts from drama to a race against time.

Other companies join the effort, rapidly developing an effective treatment. Hospitals begin to stock the new drug and patients recover as supply improves. The crisis eases as the cure becomes accessible to the public.

Jae-hyuk and his family spend time at an amusement park, savoring a moment of normalcy after the ordeal. He jokes about taking an overseas vacation and stocking up on drugstores, while his wife remains wary of lingering threats. The final scene hints at a global reach of Yeongasi.

The film closes by implying that Yeongasi may threaten other countries, leaving the audience with a chilling sense of the parasite’s reach. The final image suggests the danger is not contained to Korea, leaving a looming sense of a worldwide threat.