Medical professionals and government officials in Taiwan find themselves embroiled in a heated debate over the terrifying "Alvin virus," a contagion producing symptoms akin to influenza. Despite the gravity of the situation, the government hesitates to implement widespread preventive measures, leading many citizens to question the virus's very existence. However, certain virologists harbor significant concerns about its potential to mutate and cause profound health crises.
In the heart of Taipei, young lovers Jim (Berant Zhu) and Kat (Regina Lei) unknowingly roll into chaos as they ride their moped past a gruesome crime scene. After dropping Kat off at the station, Jim seeks refuge in a nearby cafe. The peace is shattered when an old woman, drenched in blood, with haunting black eyes and a terrifying demeanor, assaults the patrons. In a frenzy, she spits thick mucus at one customer and, in a sinister act, scalds an employee with oil before fatally disfiguring him. Amidst the chaos, her attack triggers a chain reaction, turning another customer into a violent aggressor. The woman pursues Jim into the street, where she meets her end beneath the wheels of a car driven by another affected individual. Stricken with fear, Jim races back to his apartment on foot, evading infected townsfolk. Desperate to warn Kat, he texts her to stay safe, vowing to come to her aid. However, calamity strikes as his neighbor attacks him, severing two of his fingers with garden shears—a harrowing injury that Jim hastily covers as he escapes.
Elsewhere, chaos unfolds on Kat's train, where two infected men wield knives indiscriminately, spreading fear and violence among the passengers. Among the chaos, a middle-aged businessman reveals his infatuation with Kat just before succumbing to the virus’s effects. As the delirious newly infected wreak havoc, including a horrific assault, Kat teams up with Molly (Ying-Ru Chen), who bears a grievous eye wound inflicted by the infected businessman. With the businessman relentlessly pursuing them, they barely manage to escape the train station, aided by an employee named Kevin, who forcibly seals the entrance to protect them, albeit at the cost of trapping them with the crazed assailant.
Reaching the hospital—a scene of utter desolation—Kat, Molly, and Kevin find themselves amidst a tide of injured souls. An emergency broadcast promises government intervention, but chaos escalates when an infected general fatally turns on the president during the live transmission. As panic grips the hospital, the infected manage to breach the doors, giving rise to even more fatalities within. Finding temporary refuge in a stairwell, Kat witnesses horrific fates befall Molly when she is seized by the businessman, leading to her terrible infection. Meanwhile, Jim struggles to reach Kat, stumbling through scenes of carnage while trying to suppress his own growing madness as he contacts her, only to confront disturbing hallucinations.
In a desperate bid, Kat manages to confront the businessman, leading to a dramatic clash where she crushes his skull with a fire extinguisher. She is saved by Dr. Alan Wong (Wei-Hua Lan), who has been hiding in the maternity ward, seeking a cure for the devastating Alvin virus. Alan’s chilling revelation of the virus's effects—an amalgamation of unleashed sexual and violent compulsions—sets the tone for their grim reality. He speculates that the infected’s cries come from a haunted awareness of their heinous deeds, yet they remain helplessly trapped in their impulses. When Kat discovers an abandoned infant among medical waste, Alan injects her with the virus, hoping to test her immunity—a last vestige of hope amid nightmare-like conditions.
As Kat uses Kevin's phone to reveal her location to Jim, he arrives at the hospital; a moment of hope flickers as she shows no symptoms of infection. Their relief is short-lived as Alan, who becomes a casualty himself during their escape towards the rooftop, faces Jim in a fatal struggle. Jim's infection further complicates matters, leading to a horrific revelation of his twisted desire to kill Kat, claiming that the act symbolizes love. Amidst the emotional turmoil, Kat’s laughter hints at her psychological unraveling, and she escapes just as chaos reigns. The echoes of gunfire resonate ominously, suggesting her tragic demise. In the aftermath, Jim, now mortally wounded, remains behind, succumbing to an eerie peace as a smile forms on his face—one that mirrors the collapse of their world.
