A film is being broadcast on television, and various residents of a high-rise apartment building are watching. The film's story follows several teens who trespass into a city that was deserted as a result of the outbreak. Finding the lifeless corpse of a demon, one of the teens revives it accidentally when blood drips from a scratch into the demon's mouth.
In reality, a young woman named Sally is celebrating her birthday at a party with her friends. When one of her friend receives a call from a boy named Jacob and invites Jacob to the party, she flies into a rage, screaming she does not want to see Jacob and demands everyone leave. As she sequesters herself in her room, she watches part of the film on television. Suddenly, the demon on the television looks at her through the television. The demon pushes through the television and attacks her. She is unable to escape, as her friends have locked the door, so they could prepare her birthday cake in secret.
As her friends present Sally with her cake, she transforms into a demon and attacks them, turning all but two of them into bloodthirsty monsters. The creatures' bile seeps through the building, burning through the ceiling and into other apartments and shorting out the electrical system. In one apartment, a dog licks up the bile and transforms into a vicious beast that attacks and kills its owner. In another apartment, a young boy left alone by his parents avoids Sally and her rampaging demon friends, but is poisoned by the bile and becomes a monster.
The demon boy attacks Hannah, a pregnant woman waiting for her husband to come home. She kills the demon boy, but a smaller, flying demon bursts out of his body to further terrorize her. Her husband, George, has been trapped in the elevator with another woman. As they escape through a service hatch, a security guard who turned into a demon after being scratched in the face by Sally bursts through the elevator door and infects the woman. She pursues George through the ceiling of the elevator cab. As he climbs the suspension cables above the cab, he kicks the demon away before entering his and Hannah's apartment in time to kill the flying demon with an umbrella.
Meanwhile, a group of bodybuilders led by a gym instructor named Hank have barricaded themselves in the building's underground car park, along with a group of tenants. Unable to break down the garage doors, they defend themselves against the demons with shotguns and makeshift weapons, such as Molotov cocktails. The demons eventually force their way in. Although they outnumber the demons, the uninfected are either turned into demons themselves or killed.
The infected start making their way back up the building. George causes a leak in the gas pipes that kills all the infected except Sally in an explosion. Hannah and George enter Sally's apartment, finding the two hidden partygoers. The four make their way to the roof but are stopped by Sally. She infects the two partygoers, but George dispatches them. George and Hannah lower themselves to the roof of an adjacent building, fighting and wounding Sally as they go. Inside the neighboring building, which is a television studio, Hannah gives birth to the couple's child. A blinded Sally appears and collapses, apparently dead. However, an image of Sally appears on several television monitors, running towards the viewer and presumably trying to attack them through the television as Sally herself was originally attacked. George smashes the monitors and he and Hannah exit with their newborn child.
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