NOTE: This spoiler was submitted by Jeremy.
We hear the voice of Maggie Vogel (Abigail Breslin) calling her dad. She's trapped in the city as a curfew is in effect. She urges him not to go looking for her, and she tells him she loves him.
Most of the global population has been plagued by the Necroambulist virus that turns people into zombies. Farmers have burnt down their fields and crops to prevent further contamination. Maggie's father Wade (Arnold Schwarzenegger) goes out searching for his daughter. Maggie is taken by armed officials and placed in an area at the hospital with other infected people. She has a bite wound on her arm, and the infection is going to gradually spread throughout her body. Wade is allowed to take Maggie home to settle her affairs until the virus progresses to the point where she must be quarantined. On the trip back, they stop at a store for Maggie to get sunglasses. Wade encounters a zombie and breaks its neck.
Wade brings Maggie home to his wife/Maggie's stepmother Caroline (Joely Richardson) and her siblings Bobby and Molly (Aiden and Carsen Flowers). The children are sent to stay with their aunt until Maggie's issue is dealt with. She talks to Bobby, who more or less knows what's going to happen to her.
Wade burns his field down. Maggie stays in her room, trying to contact her friends and attempting to ignore her situation. One afternoon, she cuts her finger and starts oozing black fluid. Fearing the worst, Maggie cuts off the finger and throws it in the garbage disposal. She retreats outside where she and Wade find a man named Nathan and his daughter Julia, both completely transformed from the infection. Wade kills Nathan with a hatchet, and Maggie runs away to Caroline before she sees what Wade does to Julia. Wade feels extreme remorse for what he had to do, though nobody blames him, but they do remind him of what he'll have to do with Maggie when the time comes. Wade visits the wife/mother of the zombies, Bonnie (Rachel Whitman Groves), who regrets not leaving her husband and daughter locked up, but also did not trust having them quarantined, as the doctors treated Julia like a test subject and not a person.
Wade sees a doctor who gives him three options on what to do with Maggie. One, he can take her to quarantine immediately, which he refuses to do. Two, he can give her a cocktail that they would give her in quarantine, so that at least she would do it with her father by her side and not with a bunch of strangers. The doctor states that it would be extremely painful for her up until the end. The third option is to "make it quick". Wade decides to make the most of his last days with Maggie. They bond together, making fun of Caroline's cooking and talking about Maggie's biological mother.
Maggie's friend Allie (Raeden Greer) visits her and invites her to hang out with friends. They gather around a bonfire, along with a boy named Trent (Bryce Romero), who is also infected. He says he would blow his brains out if he needed to. Maggie spends some alone time with Trent, culminating in a kiss between the two of them. At the end of the night, Allie brings Maggie home, and Allie cries alone in her car.
Maggie's eyes start to lose their color, and she starts to claim that she smells food when Caroline denies there being any sort of smell. She then goes to Trent's home to find his father holding a gun at Trent's bedroom door. The boy knows it's time for him to get quarantined, but he refuses to come out. Maggie tries talking him down as the authorities enter the house and forcibly take Trent away from his father.
Maggie's condition starts to get worse. She goes into the woods and encounters a small fox. She later runs back into her home with her face caked in blood. Maggie mentions the fox to her parents. Wade goes into the woods and finds the fox in pain, and he shoots it. Caroline urges Wade to call the authorities in to take Maggie away. A couple of officers do show up later, but Wade fights one of them. Maggie comes in and assures them that she is fine so that they won't take her away.
Wade takes Maggie to a garden that belonged to her mother where she planted white daisies. It is the last thing that Wade can show her to make her feel some sort of peace. He tells her that she is not going anywhere.
Maggie is almost completely zombified. Wade sits in the living room with his shotgun, not quite ready to use it. Maggie walks downstairs, her skin all grey and her eyes blackened. She sees her father sleeping, holding the gun. She goes over to him and kisses his forehead one last time. She then goes to the roof of the house and thinks about her mother. Maggie jumps off the roof, and the last thing she appears to see is her as a child frolicking through the flowers with her mother.