In 1944, during World War II in Ukraine, the remnants of the 5th SS Panzer Division Wiking flee from the Red Army and seek shelter in an abandoned cabin. As darkness falls, they are attacked by vampires inhabiting a hidden crypt beneath the cabin.
In present-day Sweden, doctor Annika Petra Nielsen and her teenage daughter Saga Grete Havnesköld are moving to a town in Lappland so Annika can work near the famous geneticist Gerhard Beckert Carl-Åke Eriksson. The polar night has begun, with one month until dawn, much to Saga's dismay.
Saga finds a friend in Vega Emma Åberg, an enigmatic goth girl who invites her to a party. Saga decides to go, having nothing better to do. At the local hospital, the medical student Sebastian Jonas Karlström finds some pills that Beckert has been using to treat a comatose patient. Sebastian thinks the pills are drugs and tries them. Rather than getting high, he starts to develop acute hearing and improved vision, is able to talk to dogs and tormented by extreme thirst. Vega shows up at Sebastian's house to pick up the drugs she paid him to provide for the party. Sebastian tells her he has forgotten to get them, but as Sebastian rushes to meet his girlfriend Cornelia Nour El Refai's parents, Vega finds the pills and steals them.
At the hospital, Annika looks after the coma patient, who suddenly bites her. Annika looks to Beckert for aid, but he knocks her unconscious when he realises what has happened. It turns out that Beckert is the last survivor of the massacred platoon who, after a showdown in the cabin, was left the only survivor along with a child vampire named Maria. Beckert brought Maria to northern Sweden with the intention to create a cure for vampirism, but he eventually changed direction to create a new species of human-vampire hybrids. The pills are capsules of vampire blood Beckert has used to covertly spread and study vampirism. Annika manages to free herself and fight off Beckert who turns into a monstrous vampire form. Annika drives a stake into Beckert's heart and runs him over with an ambulance, destroying him.
Vega gives the pills to John Niklas Grönberg, the party host, who consumes a pill. John puts the remainder in a bowl of punch to liven up the party, unwittingly infecting multiple guests including Vega. John eventually turns into a vampire and starts mauling his guests. As others succumb to vampirism, the party turns into a bloodbath. Saga hides in the basement, but is attacked by Vega. Saga manages to escape into the backyard, followed by Vega. Saga impales Vega on a garden gnome, who disgruntledly complains about her undignified death. The police arrive to investigate noise complaints (the neighbours mistake the vampires for drug users), and are soon overrun by the vampires. Saga is driven off in an ambulance as the vampires surround the police. John taunts them by saying > Don't worry. This will be over any minute. Dawn is... just a month away!
In the ambulance Saga encounters Maria who tells her that they will be sisters. As Saga looks to the driver seat she sees Annika looking back with red vampire eyes.
