The film focuses on the development of the relationship between Julie (Palmer), a young woman, and "R" (Hoult), a zombie, and how their eventual romance develops throughout. The film is noted for displaying human characteristics in zombie characters, and for being told from a zombie's perspective.
| ''SPOILER ALERT!!''
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About eight years after a zombie apocalypse, R, a zombie, spends his days wandering around an airport which is now filled with hordes of his fellow undead, including his best friend M. R and M achieve rudimentary communication with grunts and moans and occasional near-words. As a zombie, R constantly craves human flesh, especially brains, as he is able to "feel alive" through the victims' memories he experiences when he eats them.
While R and a pack of zombies are out hunting for food, they encounter Julie Grigio and a group of her friends, who were sent out by Julie's father from a heavily fortified, walled-off human enclave in a nearby city to recover medical supplies from abandoned buildings. R sees Julie and is drawn to her. After being shot in the chest by Julie's boyfriend, Perry, R kills him while Julie is distracted, and eats his brain, giving R his thoughts and memories, making his attraction to Julie even stronger. He rescues Julie from the rest of the pack by wiping some zombie blood on her face, masking her scent, and takes her back to an airplane he lives in at the airport to keep her safe.
Julie is initially terrified of R, and suspicious of his intentions. Julie starts to trust R after he rescues her during a failed escape attempt and finds her some food. R insists that Julie has to stay with him for a few days until he deems it safe enough for her to leave. The two bond, listening to LP records and playing games to kill the time, causing R to slowly begin to come to life. After a few days, Julie gets restless, and tries to return home, yet attracts swarms of zombies, requiring R to rescue her. After fending off a group of zombies, including M, who is confused by R's actions, R decides it is time to return her to the human enclave.
On the way, R reveals to Julie that he was the one that killed Perry, which prompts her to abandon him and return to her home alone. R begins to make his way back to the airport, heartbroken. He then sees that M and other zombies are also beginning to show signs of life, making all of them targets for the Boneys – skeletal zombies who, having lost all traces of their humanity, have shed their flesh and prey on anything with a heartbeat. R and M lead a group to the human enclave, where R sneaks inside the wall.
There, he finds Julie and meets her friend Nora, who is initially shocked. When R reveals that the other corpses have also been coming back to life, the three of them attempt to tell Colonel Grigio, Julie's father and leader of the survivors. Colonel Grigio, however, refuses to believe corpses can change and threatens to kill R, stopping only when Nora pulls a gun on him. Julie and R escape to a baseball stadium where the rest of R's group is waiting, but soon find themselves under attack by a horde of Boneys.
While M and his gang of zombies square off against the Boneys, Julie and R run, but find themselves trapped. Taking the only escape route, R jumps with Julie into a pool far below, shielding her from the impact. After Julie pulls R from the bottom of the pool, they kiss. Colonel Grigio arrives and shoots R in the shoulder without warning. Julie attempts to persuade him that R has changed, when she notices that he is bleeding from his wound – revealing that he has completely revived and is human once more. The humans and zombies combine forces and kill most of the Boneys while the rest die off from starvation and the zombies slowly assimilate into human society. Now fully alive, R and Julie watch a wall surrounding the city being demolished, signifying the end of the apocalypse.