A deadly virus spread by rainfall forces siblings Simone and Rasmus and their parents to seek refuge in a well-equipped underground bunker. The siblings’ father, Dr. Frederik Andersen, is a scientist who works for Apollon, the company that crafted the bunkers. He leaves the siblings with their mother in the bunker, saying he will be back. Before he leaves, he instructs Simone to protect Rasmus, as he is the key to curing the virus. On hearing someone attempting to get inside the bunker, Simone opens the bunker door, assuming it is her father. Her mother, knowing that it is not Frederick and that their family will be infected if anyone else comes in, runs outside the bunker and tackles the stranger before he can enter. She is exposed to the virus and quickly dies, leaving Simone and Rasmus alone. At first, the children are able to contact someone by radio, but the communication eventually stops. Simone has flashbacks to her parents' arguing about treating a sick Rasmus with some sort of virus. Six years pass. With their food supply dwindling, Rasmus wants to leave. Simone agrees, but first goes out alone in a contagion suit while Rasmus sleeps and finds only collapsed, desolate ruins. Later, back at the bunker, the air control alarms go off, signaling that oxygen is depleted. When Rasmus and Simone open the hatch to escape, they are confronted at gunpoint by other survivors who had covered the vents to force the siblings out.

Simone shows the group's leader, Martin, a map of other bunkers and promises them food. She then destroys the map, telling them they need to keep her alive to find it. In a flashback, Martin is revealed to have been a soldier tasked with holding the quarantine line at the onset of the virus. Unable to shoot a woman who is walking the highway holding an infant, Martin allows her to pass the checkpoint. He later finds his patrol dead, infected by the woman. In the present, the group encounters two men, referred to as "strangers", who are hunting survivors. Martin and Patrick (another member of Martin's group) kill the men. As the group travels, they struggle between doing what is needed to survive and holding on to their humanity. Finally, they arrive at the second bunker. Simone finds her father's phone in the bunker and learns, via information she finds on the device, that her father has likely traveled to Apollon headquarters in Sweden. Simone tells Martin that she and Rasmus are going to find their father.

Simone and Rasmus venture into the city of Copenhagen as they travel toward Sweden. They are soon joined by Beatrice, Lea, and Jean, three members of Martin's group. Lea believes that Simone's search represents hope and calls Simone "their guiding star". Emotionally connected to Beatrice, Martin later follows along with a frustrated Patrick. Separated, each finds shelter during a rain storm. Simone helps a small child, only to later see him attacked and killed by a desperate group of starving scavengers. Beatrice and Rasmus take shelter in an expensive house, with Beatrice claiming that it is the house she was born in. Martin and Patrick take shelter inside a bus. Playing with a piece of tech he took off the dead "strangers", the device activates, revealing itself to be a map. Martin's group meets up after the storm, but a scavenger follows them and takes Rasmus hostage. The scavenger threatens to slit Rasmus' throat if the group doesn't give up their food supply to him. Martin and Patrick arrive and the scavenger stabs Rasmus before escaping with a bag of food supplies. The group arrives at a third bunker, only to find it stripped bare. In a flashback, Beatrice tells Martin a story about her family house, similar to the one she told Rasmus, revealing that she is not being truthful. In the present, Martin shows Simone the map and they determine it is of the quarantine zone. Because the map shows a wall to the north, Martin concludes the spread of the virus was stopped. The wall, he declares, is where they need to go.

Rasmus, suffering from the stab wound he sustained earlier, is unable to walk, so Simone plans with the group to steal a car from one of the quarantine patrols run by the "strangers". The group manages to successfully intercept a quarantine patrol and incapacitate a group of "strangers". As Martin interrogates the captured "strangers" for information, Jean spots a snake tattoo on one of the men and recalls a memory that haunts him. He urges Martin to kill the men, and Patrick agrees. Martin seemingly does so, upsetting Simone. In a flashback, Jean stays with a kind family in the early days of the virus. One day, a quarantine patrol arrives and a group of "strangers" confronts the father of the family, instructing him to come with them. When the father refuses, he is killed by a man with a snake tattoo and the mother is taken away. Trying to quiet the shocked daughter to protect them from discovery by the "strangers", Jean accidentally smothers her to death. After the "strangers" leave, Jean buries the family and is introduced to Beatrice and Lea, who had come upon him as he stood before the graves he had just dug. In the present, the group has moved north and meets a doctor who operates on Rasmus’ stab wound. After Simone reveals who their father is, the doctor tells them she, too, used to work for Apollon. The doctor lures Simone and Rasmus away from the rest of the group and to her own bunker, confessing that she blames Dr. Andersen, the siblings’ father, for the death of her children. The doctor attempts to inject Rasmus with a fatal poison. The "strangers" arrive and, seeing the man with the snake tattoo, Jean shoots him in the head and is then captured by the rest of the "strangers". The others flee to the bunker. Patrick goes in alone and shoots the doctor, declaring that this is how things will be done from now on. Later, Patrick asks Martin why he listens to Simone and not him. Martin tells Patrick that the latter made the choice not to help Jean, and so is partially responsible for his fate. Patrick breaks down in tears. The group takes shelter as the rain falls during the night.

Lost in the woods, the survivors stumble upon a mansion where residents invite them to stay the night. Martin is distrustful of the residents, who appear very cult-like. The others enjoy the accommodations, including showers, fresh food, and clean clothes. They are asked to forget their past and live in the now. In a flashback, Lea, a devout Christian, goes to a teen party against her mother's wishes. As a prank, her school peers spike her drink, and Lea's drunken behavior is caught on video and shared. Her mother sees the video, which has gone viral, and disowns Lea. Inside the house of the party, Lea prays for God to save her, and moments later, the rain begins. Her partying peers die from the virus (having all gathered outside except for Lea herself). Lea's mother dies from the virus also, having stepped outside during the rain in an attempt to pick Lea up. In the present, Lea bonds with an older woman, Karen, who teaches her to let go of the guilt she carries. Simone discovers that an Apollon employee is a resident of the mansion and seeks information from him. That night, all residents of the mansion are invited to a monthly "ceremony". It is revealed that every month, a member is randomly chosen to be killed and eaten for the next month's ceremony, allowing them to be one with the group. Lea is "chosen", but Karen takes her place and the survivors are allowed to leave. Because he will not let go of the past, the Apollon employee is forced out of the mansion against his will. When he catches up with the survivors, he tells them Apollon is responsible for ending the world, before injecting himself with the virus and dying. Simone collects the syringe and the group walks into the night.

As the group squats in a cabin, Patrick is hurt when Martin states the two are not friends during a casual conversation. Needing supplies, the group prepares to go to the next bunker, but Rasmus says he is too sick to move. Simone, Martin, Patrick, and Lea go, planning to return with supplies. It turns out that Rasmus faked his illness to spend more time with Beatrice. It rains and the four are stuck in the bunker. They find videos of Apollon testing vaccines on random individuals. Depressed and alone, Patrick gets drunk. In a flashback to before the rain, Patrick is a loser who cannot hold a job or a girlfriend and his father disowns him. He is getting high in his car when the rain comes, sparing his life. In the present, a drunk Patrick tries to kiss Simone. When she rejects him they tussle and Patrick pushes her into the rain. Patrick tells Martin to shoot her, assuming she is infected. Martin hesitates, and Lea stands between the gun and Simone. Lea then backs up into the rain with Simone. When neither has a reaction, the group realizes the rain no longer causes infection. Back at the cabin, Beatrice and Rasmus enjoy their intimate day together, but a leaky roof causes Beatrice to be hit with a drop of water. Rasmus kisses her, assuming she's now infected, and they resign themselves to dying together. In the bunker, Martin tells Patrick he needs to leave and not come back, because he had pushed Simone into the rain. In a flashback, Patrick meets Martin for the first time, Martin telling him he's lucky they met when Patrick gives him a lighter. Patrick says it is the first time anyone has said that. In the present, Patrick returns to Rasmus and Beatrice, only to find Rasmus in a state of panic carrying Beatrice’s corpse—she had come into contact with a stray, infected dog seen earlier near the cabin. Patrick panics and runs away, only to trip and be captured by the "strangers".

A band of armed "strangers" tortures Patrick in order to extract information from him. Patrick discloses to the "strangers" the location of the bunker, where Simone and Lea and Martin are, and about Rasmus not having been infected by Beatrice. Meanwhile, Rasmus arrives at the bunker, carrying Beatrice's corpse. In the bunker's quarantine room, Rasmus begins to unravel, refusing to leave Beatrice's side. Eventually he does, but he contacts the "strangers" and allows himself to be captured. In a flashback, Sten, the head of Apollon, briefs the mission to the "strangers", and then reveals to them that they have ingested nanocapsules of the virus which will be triggered if they leave the zone. Like the survivors, the "strangers" are trapped. In the present, the leader of the "strangers", Thomas, feels that Rasmus is the one they are looking for. After burying Beatrice, Simone, Martin, and Lea capture a "stranger". They try to trade him for Rasmus, but a depressed Rasmus takes the syringe with the virus and injects himself. However, he does not die. Martin makes up with Patrick, setting him free. Everyone leaves for Apollon headquarters, hoping Rasmus is the cure. As they leave, a "stranger" kisses Rasmus on the forehead for good luck. He dies of the virus soon after the rest of the group is gone.

Arriving at Apollon headquarters, the group is separated. Rasmus and Simone meet their father while Martin, Patrick, and Lea wait in a bunker. There, they are reunited with Jean, much to Lea’s joy. Simone's father, Dr. Andersen, tells her that they need to get away. To get the antidote, he tells her they will need to access Rasmus' brain and bone marrow, killing him. In a flashback, Dr. Andersen lies to Apollon, claiming his children are dead in order to save them. Dr. Andersen has been exposing people to the virus in an attempt to find someone to replace Rasmus, and brutally killed a colleague who found out the lie. In the present, a doctor taking a swab from Rasmus is infected and dies. Dr. Andersen realizes the virus has mutated and that Rasmus is now a carrier, needing to be stopped to save the world. As the lab goes on lockdown, Lea, Jean, Martin, and Patrick are reunited with Simone and Rasmus, and they plan to flee. However, Dr. Andersen shoots Martin in an attempt to stop the group from leaving with Rasmus. Patrick arrives with a stolen vehicle and the group reaches the wall. The "strangers" block their path, and Thomas tells them that the nutritional supplements Jean, Lea, Martin, and Patrick were given contain the virus nanocapsules. The four cannot leave, except Simone with Rasmus. The group encourages her to go. However, Simone cannot bear leaving her friends behind so the group backs away from the wall and drives off. Thomas tells Simone that he'll be after her. In a final scene, Sten is making a sales pitch to a select group of investors, telling them the virus can be controlled and weaponized.

Following the lab disaster, Simone arrives and convinces Rasmus to lie and say someone dropped a sample, infecting them all. Unbeknownst to them, Jakob's sickly younger sister Sarah witnessed her brother's death and their coverup from outside the lab. Martin accompanies Simone on a journey to retrieve her father's computer. At the base, Patrick stumbles upon a hidden room filled with strange devices.

As Apollon surrounds the base, Simone scrambles to find a cure. Rasmus's secret draws Sarah to him—but threatens to tear the group apart.

Simone is rescued by siblings Daniel and Luna who, with their parents, are living off the grid. Back at Apollon, Rasmus convinces Sarah to be studied by Sten so that they can spread the virus and create a new human race. Martin escapes imprisonment and frees Fie and Patrick, but they refuse to leave due to Fie's impending labor and their belief that Simone is dead. Fie and Sarah reunite but Sarah is resentful of Fie leaving her for dead, while Martin is recaptured. Daniel and Luna's father wants Simone gone as soon as she recovers as he fears her discovering the flower extract. Luna, however, demonstrates to Simone the power of the flower extract to repel the virus and she eventually follows the father to the source. Rasmus attempts to convince Martin that Simone is dead by showing him the tracker and asking him to join him; while Martin initially refuses, when alone, he apologizes to Simone for not being able to run anymore. Fie rejoins Apollon as a biologist after discovering Apollon was right as Sarah is living proof; she discovers a person's immune system must be completely compromised to absorb the virus. Sarah and Rasmus discover many of the terminally ill patients intended to receive the virus are already dead as Sten has started testing. Simone is nearly executed by Daniel's father to protect their secret; however, Daniel attacks his father and allows her to escape. Martin agrees to join Rasmus and receive the virus.

Simone, Kira, Jean, and Daniel go after Luna, who has been kidnapped by the young scavengers, intending to trade food for her return. However, the man leading the group tricks Simone into using the flower extract to escape the virus after being attacked and cornered by the young scavengers. He demands the flower extract in return for Luna. Martin discovers Simone's survival after Patrick attempts to keep it hidden, and Rasmus allows Martin to leave to pursue Simone since he has forcibly taken control of Apollon following Sten's death. Rasmus, who has developed a lesion on his neck, uses his powers to spy on Martin. Sarah is resentful of him attempting to bring Simone back but worries about the lesion. Simone brokers a deal with the leader of the young scavengers, but he double-crosses them and spikes their Coca-Cola in order to go after the flower. Rasmus infects more of the sick patients but weakens during the process, worrying Fie, who confides in Sarah. Rasmus overhears and becomes convinced that Fie should be the next infected since he believes he cannot trust her. Fie and Patrick make the decision to flee Rasmus and Apollon. Jean frees Simone, Kira, Daniel, and Luna, but they are too late as the flower's defenses killed almost all the young scavengers (except the one shot by Kira) and their leader. As a result, the flower is dead and only one vial of extract remains. Martin finds Simone in time to see Daniel and Simone kiss; Rasmus also sees this as he is telepathically spying on Martin.