Sam (Anders Danielson Lie), a Scandanavian musician living in Paris, visits his ex-girlfriend Fanny (Sigrid Bouaziz) to recover music tapes left in her possession. Fanny is holding a party and brushes Sam off multiple times before telling him where to get the tapes. Sam is accidentally bumped by a party-goer on his way to the tapes. Once there, his nose starts to bleed and he passes out. Sounds of chaos erupt outside the door of the office, but Sam sleeps through it.The next morning, Sam wakes to find the flat trashed, with blood stains on the walls and no one inside. He ventures into the stairway and discovers a zombified Fanny who charges at him when he calls her name. Locking himself in her flat, he witnesses the death of a family from the apartment across the street who attempt to escape in their car. The flat, which is several stories up, is the only safe location that Sam can find. An army of flesh-eating zombies have seemingly taken over all of Paris, are very fast moving, and respond in hordes to any sight or sound. They are also completely silent, making almost no noise and never vocalizing.Unable to leave the flat, Sam begins cleaning up until a shotgun blast erupts through the floor. Looking through the hole, he discovers that one of the residents below has committed suicide, after killing his wife, who he had bound to a chair after she become infected. He retrieves their shotgun and ventures outside, finding the zombies in the stairway gone. He quickly closes the doors to the building to seal himself inside. Sam explores the building's apartments one by one, finding most of them empty. He finds a zombified elderly man (Denis Lavant) in the building's elevator, binds the gate up, and begins conversing one-sidedly with the zombie whose name he learns is Alfred.Sam successfully raids the building for a large quantity of food and supplies. He rations these and discovers musical equipment in one room, which he uses to entertain himself. As time passes, Sam becomes increasingly lonely and unhinged. Desperate for company, he attempts to capture a stray cat wandering aimlessly among the un-dead, but is nearly killed by several zombies. He makes it back to the apartment. In an engaged state, Sam shoots the cat from the window. Fearing he was bitten, Sam nearly kills himself a second time when he falls asleep with the shotgun placed beneath his head while waiting to see if he would turn.As winter approaches, he is forced to contend with a lack of heat, and the running water supply to the building finally stops working. Surviving by collecting rain water and creating a wood-burning fireplace, Sam's mental state continues to decline. He notices one day that the streets are largely empty. Sam decides to test at how far away the zombies are by loudly playing a drum set. The un-dead return in hordes and nearly climb into the flat by piling atop one another. An enraged Sam continues to play despite the danger, his mind beginning to snap.That night, Sam hears movement outside his bedroom door and fires the shotgun through it. He hears cries of pain and realizes he shot another person. He attempts emergency medical aid and finds the woman's bag, which has a large amount of rope and a grapnel hook she uses to go from rooftop to rooftop. The next morning, Sam speaks with the wounded woman, Sarah (Goldshiften Farahani), who explains how she has survived. He retrieves supplies from the lowest flat and kills the zombified occupants. She goes to the roof and tells Sam he will either die or go insane if he does not leave the apartment and goes on the road with her. Sam at first brushes her off, but relents. He returns to his room and finds Sarah dead from the gunshot wound.... their entire conversations were a result of hallucination. He mourns her loss, eulogizes her, and covers her body.Sam decides to leave the apartment, burning the tapes and releasing the zombie Alfred, who wanders into his nearby flat where Sam locks him away. The burning tapes set off a fire alarm. As a result the zombies attack the building. They break down the doors and rush inside. Killing several with shotgun blasts to their heads, Sam manages to get to the roof and swings across the street to the next building, where he climbs to the rooftop and stares out into the seemingly endless skyline of Paris.