Medical experts and government officials in
Taiwan argue over the "Alvin virus", a virus responsible for an infection similar to
influenza. The government has refused to take large-scale measures to prevent the transmission of Alvin, and many ordinary people doubt that the virus exists at all. However, some virologists fear that it has the potential to mutate and cause serious illness. Jim and Kat, a young couple in
Taipei, wake up one morning and discuss a miscommunication about a trip they had planned. Jim then goes to his balcony and sees an old woman on a neighboring roof that is covered in blood; she is gone by the time Kat appears, ready for work. As he drives her to the train station, they pass a bloody crime scene. Jim drops Kat off at the station and then goes to a café. The same blood-soaked old woman now enters the café with black eyes and a crazed expression and assaults other customers, spitting thick mucus on one of them and burning an employee with hot cooking oil before tearing off his face. The man she spat on suddenly attacks his own brother with a knife. The woman chases Jim into the street, where she is hit by a car driven by a man with the same symptoms as her. Jim flees back to his apartment on foot, chased by the infected locals. He texts Kat to stay where she is, promising to rescue her. His infected neighbor then attacks him, cutting off two of his fingers with
garden shears. Jim hits him over the head and manages to escape, using a cloth to cover his injury. He then jumps on his moped to go after Kat. Meanwhile, Kat is harassed by a timid and lonely middle-aged businessman who becomes frustrated when rejected and scolded by Kat. As more passengers board the train, Kat offers her seat to another woman, Molly. Suddenly, an infected man begins stabbing passengers indiscriminately, quickly creating a horde of infected people. Molly and Kat are cornered by the now infected business man who stabs Molly’s eye with an umbrella. Kat and Molly escape the train, but Kat loses her phone. The businessman pursues them until they reach a young bodybuilder who tries to defend them. The businessman is disarmed by the bodybuilder but retaliates by biting part of his nose off and killing him with a fire axe that he grabs from one of the station walls. Kat and Molly barely escape the train station as a lonely employee named Kevin shuts the entrance. Kat admonishes him for leaving them to be killed by the businessman, to which Kevin becomes defensive, stating that he was just following police orders. Kat, Molly, and Kevin arrive at the hospital, which is overrun with the injured. A nurse examines Molly and transfers her to a wheelchair for treatment. Kevin inquires about his injured nose and is told by the nurse that since it is not broken, he will be fine. Kat asks Kevin to use his cell phone to contact Jim. The government broadcasts an emergency message vowing to take control of the situation, but an infected army general kills the president with a grenade on live TV. The men in the hospital begin to panic and fight each other, distracting police officers just as the businessman and other infected people break through the doors. Kat escapes into a stairwell as the businessman finds Molly in a wheelchair and rapes her empty eye socket, infecting her as Kevin hides under a gurney and listens to Molly's screams. Jim escapes to the outskirts of town, passing by scenes of violent behavior along the way. He contacts Kat, who tells him where she is. As they talk, he begins to cry and hallucinate that a discarded mannequin head is lapping up his blood and sweat. Kevin tries to exit the hospital but is discovered by an infected Molly and is killed by her and the horde. The businessman continues his pursuit of Kat through the hospital's hallways, but she ambushes him and smashes his head in with a fire extinguisher. She finds safety from other infected people by Alan Wong, a virology specialist who has been hiding in the maternity ward. Alan explains that he attempted to find the cure for the Alvin virus, which, in its mutated form, connects the parts of the brain that control sexual urges and aggression. He theorizes that the infected cry because they are aware of the terrible things they do but are unable to stop, likening it to resisting the urge to blink. When Kat finds an infected baby in a medical waste bin, Alan injects her with the virus to see if she is immune to it. He reveals that he conducted similar tests on the babies who had been abandoned in the ward, but all of them had become infected, and he was forced to euthanize them. He says that he will kill Kat if she becomes infected, but will spare her if not, as her immunity would hold the key to stopping the virus. Kat uses Kevin's phone to text her location to Jim, who has just arrived at the hospital. When she exhibits no symptoms of the virus, Alan frees her and calls for a military helicopter to escort them both to a safe location, warning her that the soldiers will not rescue her without him. As they make their way to the roof, two infected people attack them, cutting off part of Alan’s foot and vomiting in the wound. Alan manages to shoot and kill the infected and screams at Kat to save him. An infected Jim arrives. Alan attempts to shoot Jim, but the gun backfires, giving Jim the chance to strike Alan in the neck with the axe. With his last breath, Alan states that he enjoyed euthanizing the infected infants before succumbing to the virus. Kat steals Alan's keys and locks Jim out of the stairwell leading to the rooftop. When she attempts to talk to the uninfected part of Jim, he tells her that the infection has given him purpose, promises that he still loves her, and begins detailing how he intends to torture and kill her. Kat begins laughing hysterically before fleeing up the stairs and through a door leading to the rooftop. As the door closes behind her, the sound of a helicopter can be heard. Mortally wounded by Alan, Jim sits against the stairwell gate and dies with a grin on his face as gunshots sound, and Kat can be heard screaming in pain. == Cast ==