Plot
A family of four, Dino, his wife Pernille and their children, the young Maj and teenager Gustav are living in idyllic Sorgenfri, Denmark during the summer. Pernille dotes on Maj, who in turn dotes on her pet rabbit, Ninus. Another teen, Sonja, moves in with her mother across the street, much to Gustav's interest. At a picnic one of the neighbours, Casper, tries to persuade Dino to get a gun so they can go hunting together, but Dino refuses. Meanwhile, some picnickers, specifically a little boy, is seen throwing up, possibly teasing at the fact that he might be infected. An elderly neighbour comes running to the party and says her husband has died, but upon investigation his body has gone missing.
Over the course of several days, the situation escalates as a deadly virus that reanimates its hosts into hostile killing machines sweeps the neighbourhood, but Dino lies to the children assuring them that everything is alright. Sorgenfri is then quarantined by the military which orders all residents to self-isolate. The military keeps the families supplied but each house is kept strictly incommunicado, and wrapped with plastic sheets, and the power frequently fails. A news report indicates that "More than a month's research into the virus has yielded no results."
During the night some of the neighbours are dragged from their homes and placed in the back of trucks and transported to a "safe space", as the military tells them. Gustav sneaks out to investigate, and first checks in on Sonja and sees her tending a wound on her mother's leg. He makes his way to the "safe place" and finds that it is actually filled with infected and that they get fatally shot by the military. He decides to open the back of the trucks to free the patients, and is discovered by the military. Gustav flees, not knowing that he has unwittingly freed the infected. The military is quickly overrun and retreats, abandoning everyone to their fate. Sonja and her infected mother are taken in at Gustav's insistence against Pernille's protests.
As zombies overrun the neighbourhood, Casper's significant other Anna escapes the house and flees in their car after having made several attempts. Abandoned, Casper joins Dino and prepares for a siege while Sonja's mother grows progressively sicker and the pet bunny Ninus has gone missing. At breakfast, everyone is eating some kind of stew with meat in it and Maj comes in, asking about her pet rabbit Ninus. Dino and Casper explain that rabbits are good runners and Ninus has possibly run away, though the scene implies that the stew that the family is eating is actually Ninus. The infected begin roaming the night and Gustav's habit of peeping with the lights on nearly gets them all killed. Anna eventually returns, thoroughly traumatized. Sonja and Gustav have a romantic interlude and afterwards she returns to her room to sleep with her mother. She checks on her mother and notices that her condition is rapidly deteriorating. When Sonja wakes up in her mother's embrace, she slowly realizes that her mother is in full rigor mortis.
Dino and Casper scout the area, finding an abandoned landscape, except for dead neighbours, surrounded by a wall. After fair warning, and almost refusing to budge, Dino and Casper are fired upon by the military and forced back. While in another romantic interlude, Gustav and Sonja are caught red-handed by Pernille, and told that Maj is missing. Gustav and Pernille go outside to look for her and find corpses in the street while. Dino and Casper enter a grocery store and come across a woman with a box of food and she asks if they have seen her missing boyfriend. They hear a noise and Dino looks through a crack in a door and sees a disemboweled zombie. Casper brings up his rifle and just as he goes through the doors he is pulled away and begins screaming. Dino goes through and picks up Casper's rifle, but immediately sees a horde of zombies coming his way. Casper is killed and Dino and the woman flee before they are overrun. Dino then robs a woman of her meagre supplies at gunpoint even though she protests of her having a family and kids.
Meanwhile, Sonja and Anna are alone in the house and hear a noise upstairs as Sonja's mother reanimates.
Maj, having previously snuck out to find Ninus, sees the neighbour whose husband's corpse went missing at the start of the movie and Maj asks her if she has seen Ninus, only for the neighbour to turn around and reveal that she is actually infected, with a hole through her cheek so that her teeth are visible. The neighbour then proceeds to pounce onto Maj and bites her, quickly infecting the young girl. Her family hears her screams and quickly rush to her safety, killing the infected neighbour with a gunshot. The gunshot attracts a horde and the family retreats. Previous preparations to the house prove adequate against the approaching zombies for the time being. Upon their return, Anna has turned, and Gustav finds Sonja standing over her downed mother.
Dino says they have to kill Maj before she turns, but Pernille takes her and hides in their bedroom. Dino breaks down the door to kill Maj but Pernille refuses to let him shoot her daughter, to which Maj turns into a zombie and proceeds to bite into her mother's cheek and neck, possibly killing her. Dino then hesitates to kill his daughter, so he pulls the rifle on himself, putting it under his jaw and pulling the trigger, only for the gun to click empty. Maj then proceeds to pounce onto Dino and kills him mercilessly as the scene ends.
Gustav and Sonja barricade themselves in his room, but find Sonja's mother was not actually dead. They finish her off, and then use a firework to distract the zombies. They wander into the horde, and as the firework begins to fade the zombies become aware of their presence, but they nonetheless succeed in escaping through the forest.
The camera pans upwards and outwards over the forest, revealing stems of smoke coming from several places in the town. The sound of a Danish air raid siren can be heard, along with the whistling of a bomb being dropped. At the height of the whistling's crescendo a deep boom simultaneously occurs as the screen cuts to black, and begins to flash the words "WHAT WE BECOME" across the screen repeatedly, then the end credits begin to roll.
