A family of four—Dino, his wife Pernille, and their two children, the young Maj and teenager Gustav—are enjoying a peaceful summer in the picturesque Sorgenfri, Denmark. Life seems blissful until the arrival of a new neighbor, Sonja, who captures Gustav's attention. While Pernille showers affection on Maj, who is deeply enamored with her pet rabbit Ninus, a nearby picnic takes a dark turn. Here, a neighbor named Casper tries to convince Dino to accompany him on a hunting trip, but Dino refuses the temptation. Tension rises when a little boy at the picnic is seen throwing up, hinting at a lurking illness. Soon after, an elderly woman bursts in, distraught over her husband's mysterious disappearance.
As the days unfold, their idyllic life shatters with the onset of a deadly virus that transforms its victims into ravenous killing machines. Despite the growing danger, Dino reassures his children that everything is fine. The military steps in, enforcing a quarantine that keeps the families isolated and supplies limited, all while power outages create additional fears. As reports indicate that the research into the virus has yielded no results, panic ensues.
One night, neighbors are mysteriously taken from their homes by the military, supposedly for their safety. Driven by curiosity, Gustav sneaks out to check on Sonja and discovers her mother injured. When he investigates the "safe space," he finds it filled with the infected, brutally shot by the military. In his attempt to liberate them, Gustav inadvertently unleashes a horde of zombies, leading to the military’s retreat and leaving the remaining residents to fend for themselves. Against Pernille's objections, Gustav insists on taking in Sonja and her ailing mother.
As the zombie invasion escalates, Casper's partner Anna makes a daring escape from their home. Casper and Dino prepare for a siege while the family faces their own horrors, including the unsettling absence of Ninus. The atmosphere of dread thickens when Maj asks about her beloved rabbit during breakfast, and though her father tries to comfort her, hints suggest that the meat in their stew might be Ninus. With monsters roaming outside, their safety is further threatened when Gustav's tendency to peek attracts deadly attention.
Amidst the turmoil, Anna returns, haunted by her experiences. Tensions flare when Sonja and Gustav share a fleeting moment of intimacy, only to discover that Sonja's mother's condition has worsened. Gradually, Sonja comes to the horrifying realization that her mother is in rigor mortis.
Dino and Casper attempt to scout their desolate surroundings but are met with aggression from the military. In a shocking twist, they run into a woman looking for her missing partner, only to find themselves in dire circumstances with a Zombie ambush. The chaos continues as Maj ventures out to find Ninus, confronting the neighbor whose husband went missing. In a tragic turn, Maj is bitten, and despite her family’s desperate attempt to save her, the sound of a gunshot alerts the zombies.
Now cornered, the family strives to fortify their home, only to return to find that Anna has succumbed to the infection, and chaos boils over. As Dino faces the grim reality of having to kill his daughter Maj before she becomes a zombie, the scene spirals into a gut-wrenching climax as Pernille refuses to let go, leading to a horrific confrontation that seals their fates.
Gustav and Sonja barricade themselves in a room, finally confronting Sonja's reborn mother. Using fireworks as a diversion, the pair attempts to navigate through the chaos, barely escaping into the forest as the remnants of their world collapse around them.
The camera pulls back, revealing the smoke rising from their town, accompanied by the piercing sound of an air raid siren and the ominous whistling of an impending bomb. As tension builds, the screen fades to black, flashing the words "WHAT WE BECOME" while the end credits roll, leaving the audience to grapple with the haunting aftermath.
