The Sadness (2021)

Jim and his girlfriend Kat are preparing for their usual day in an apartment complex in Taipei. Online, he sees that medical experts and government officials in Taiwan continue to argue over the "Alvin virus", responsible for an infection similar to influenza. A virologist, Dr. Alan Wong, warns that the virus has a high potential for mutation. The government, prioritizing their re-election campaign, are downplaying the viral threat, while most of the general public expresses skepticism. Jim spots an elderly woman covered in blood on a nearby rooftop, but she vanishes when he looks away.

As Jim drives Kat to the subway station, they pass by an ambulance with a bloody corpse on the stretcher. After dropping her off, he stops by a cafe, where the same elderly woman appears. She viciously attacks the cook with a frying basket, and spits mucus onto a customer who tries to intervene, causing the man to turn on and kill other nearby patrons. The woman chases Jim, but is run down by an infected driver. Jim flees back to his apartment, witnessing chaos in the streets caused by the infection. While trying to get in contact with Kat, Jim is attacked by their infected neighbour, Mr. Lin, who cuts off two of his fingers with hedge clippers. Jim knocks Lin unconscious and escapes.

On the subway, Kat is sexually harassed by an older businessman, causing her to switch seats with a student named Molly. An infected man boards the train and begins stabbing indiscriminately, killing and infecting several other passengers. The infected Businessman stabs Molly in the eye using his umbrella. Kat drags an injured Molly out of the station, pursued by the Businessman, who uses a fire axe to kill a bystander who tries to defend the two. They barely manage to escape as the subway employee, Kevin, closes the door shutter on them. Meanwhile, Jim continues trying to reach Kat, witnessing further violence as he drives out of the city. After bandaging his injured hand, and arming himself with a billhook, he tries to intervene when he sees a group of infected boys beating and torturing a man on a game court. The "victim" turns out to also be infected, the group engaging in a sadomasochistic game, and Jim is chased off.

Kat, Molly and Kevin reach a hospital – filled with survivors and police officers – where Molly's eye is treated. Kat takes Kevin's phone, using it to message Jim about her whereabouts. They witness a televised address by the President and the Chief of the General Staff, promising to combat the infection, but the General reveals himself to be infected and sets off a grenade. In the hospital waiting room, a brawl starts between the president's supporters and those unsympathetic to his death. Just as the police defuse the situation, Kat spots the Businessman approaching the hospital doors, and slips out of the room before he kills one of the officers. Infected break into the waiting room, massacring or infecting everyone inside. Kevin survives by hiding underneath a cart, and watches in horror as the Businessman infects Molly by raping her injured eye socket. Jim contacts Kat, telling her to stay where she is while he makes his way to the hospital. However, Jim has begun to experience early symptoms of infection. Kevin tries to escape the hospital, but is spotted by Molly and held down by a horde of infected as she savagely kills him with a bone saw.

The Businessman pursues Kat, who blinds him with a fire extinguisher before using it to bludgeon him to death. Kat is let into a secure room by Dr. Wong, who has taken refuge in the hospital's maternity ward. Holding Kat at gunpoint, he forces her to strip and shower to remove any traces of infected blood. Wong explains to Kat that the mutated Alvin virus hijacks the limbic system, creating an irresistible desire to act out one's most violent, sadistic impulses while retaining their intelligence and speech. Wong also reveals that he has been in contact with the military, who are sending a helicopter to extract him from the city. Wong gives Kat fresh scrubs to wear, but when she disposes of her ruined clothes, she finds an infected infant in the biohazard bin. Wong suddenly injects Kat with a sample of the virus and handcuffs her. He explains that he used the newborns to conduct human trials, attempting to find an antibody for the virus. When none of the babies proved to be immune, Wong killed them. He tells Kat that if she hasn't become infected within five minutes, she is likely immune, and he will bring her with him to be evacuated.

When Kat doesn't succumb to the infection within his time limit, Wong prepares to take her to the hospital's helipad. Jim confronts them outside the maternity ward, now fully lost to the infection. Wong shoots him in the neck, but his 3D-printed pistol explodes on the second shot, allowing Jim to kill him. With his dying breath, Wong expresses that he enjoyed euthanizing the infected infants. Kat locks herself behind the metal gate in the stairwell to the roof, keeping Jim at bay. When he seems to plead with her, Kat asks him how the infection feels. Jim, dying of his injuries, says that the virus has given him purpose, and expresses his "love" for her as a desire to torture and kill her. Kat breaks down, laughing hysterically as she runs up the stairs toward the helipad. The camera pans to Jim's corpse, still grinning, while gunshots sound and Kat screams off-screen.