NOTE: This spoiler was submitted by Jeremy.
The zombie known as R (Nicholas Hoult) wanders through the airport inhabited by dozens of other undead people. Through his thoughts, he describes his predicament - he's really neurotic and can't remember much from what happened before he died, or even how the zombie epidemic happened. As he passes other zombies, he imagines what they were like before dying, like a man being a janitor, or a woman being a trainer. He shows us the "bonies" - vicious skeletons that are more violent than a regular zombie, as they decomposed to that state after so long.
R goes to his home, which is an abandoned plane. He puts on a record and listens to some music. He goes to the airport bar and meets with his friend, M (Rob Corddry). They grunt and groan, but they manage to tell each other they're hungry and to go into the city. They head off with a pack of zombies into civilization.
Beyond the zombie area is the city of the living. Julie Grigio (Teresa Palmer) goes with her best friend Nora (Analeigh Tipton) and boyfriend Perry (Dave Franco) through a zone where they watch a video of Julie's dad, General Grigio (John Malkovich) instructing survivors on what to do if they see a zombie - shoot them in the head.
As R and the other zombies get near, Julie and her group go into a lab to look for supplies. The zombies enter and start attacking. Perry shoots R, provoking him to attack. R says that he doesn't like eating people, not just because it's wrong, but also because his victim would come back as a zombie, unless he were to eat their brain, and then he'll gain their memory. As he kills Perry, he eats his brain and starts to see flashbacks of Perry's childhood, including him riding a bike or being with his father.
Julie shoots a zombie that almost gets Nora, and that's when R sees her for the first time, and he instantly feels something for her. She throws a knife at his chest, but he keeps coming toward her. He smears blood on her cheek and manages to speak a few words, telling her it's not safe. He gets her past the zombies by pretending she's also dead. He ends up taking her to his plane where he gives her a blanket and lets her sleep in a seat. She's still really scared but doesn't panic.
R goes to a car and continues eating Perry's brain. He sees more flashbacks of him being with Julie in their relationship, as well as a dinner he had with her and Grigio sometime after the outbreak. Julie's not happy with his building a barrier between them and the rest of the infected world.
Sometime later, the couple are walking when they see Perry's dad, who is a zombie. He and some other zombies run to attack them, and Julie shoots Perry's dad in the head. R realizes why she's scared of him.
The next morning, Julie tells R she's hungry. He tells her to stay there while he looks for food, but she runs out anyway. She sees zombies all around, but R gets her and sneaks her into the airport again by playing dead. She grabs a can of fruit cocktail and feasts on it. She's still amazed at R's ability to speak, but he still can't remember his name. Since he hints that it began with an R, Julie decides to just call him R. At this point, R's heart is able to start beating again.
Julie wants to leave, but R tells her to wait a few days so it'll be safe. During that time, the two bond through activities such as driving, listening to more of R's records, and dancing, not knowing that M can hear the music coming from the plane. One evening, she talks about her feelings regarding the whole aftermath of the epidemic, such as losing Perry and her mother. She feels conflicted.
As she goes to sleep, R goes back to eating Perry's brain. He sees that Perry joined Grigio's squad after his father's death, leading up to the moment R killed him. He spits the brains out and goes back to see that Julie has left again.
Julie ends up in the airport where M and two other zombies try to attack her. R comes to her rescue and fights them off. He and M try to communicate, with M telling R that they should eat Julie since she's alive. That's when some bonies come in and chase after R and Julie. M ends up coming back and hitting the bonies with a cart. After some hesitation, he gets Julie to join him and R.
They drive into the parking lot where R crashed the car they were driving. A bigger group of zombies appears, but when Julie takes R's hand (a gesture she wouldn't even do with Perry), the other zombies let them through, and they drive off.
R and Julie end up driving into a deserted town that Grigio had evacuated. They go into a house to look for food and a place to rest. Before she goes to sleep, Julie tells R that he's a good person. This makes him feel guilty, so he reveals that he killed Perry by showing her his watch. Julie says she sort of figured R did it, even though she hoped he didn't.
R then falls asleep, which he says the dead can't do, and he has a dream. He's walking through a sunny meadow with flowers and trees blooming. In the middle of the area is Julie, Nora, and Perry all talking about stuff they'd like to do. R comes up, and it appears the trio notice them. Julie asks R what he'd like to do, and, with normal speech, he says he doesn't know.
Back at the airport, M sees a picture on the wall of a couple holding hands. This triggers a memory of him when he was alive, walking with his girlfriend. He asks another zombie if he can feel something, and he nods his head. We see that their hearts are also starting to beat again. Then, a bunch of bonies come up to them. They don't attack, but they screech in M's face.
He wakes up to discover that Julie has left...again. He runs out of the house, completely devastated. We see that she's driven as far as she can until she runs out of gas, and she comes across the city where she reunites with her father.
R walks alone as it starts raining. He feels cold, which confuses him since dead people shouldn't feel that. He sees M and a big group of zombies come up, and R goes and hugs him. They go under a bridge and M tells R that the bonies are coming for him and Julie. They also realize that they are feeling more alive. R asks the other zombies for help, and they all grunt and groan while nodding their heads, which M translates as saying "Fuck yeah".
Julie is staying with Nora, and she admits she misses R. Nora thinks she may be falling for a zombie, but Julie denies it. Nora goes to bed, and Julie goes out on the balcony to find R down there. Although she's happy to see him, she told him that it was a dangerous move on his part, since anybody else would automatically shoot him. Nora comes out and sees R, and they take him in.
R tells Julie that they (the zombies) are all changing, and they need to show it, but both Julie and Nora know that Grigio would not listen to reason. They decide to put some makeup on R to make him look more human.
As they get into the city, Julie comes across her father and tries to convince him that the zombies are curing themselves. He doesn't buy it, so R tries to prove it himself. Grigio pulls a gun on him, but Julie defends him. Before he pulls the trigger, Nora holds a gun to Grigio, letting R and Julie run.
They run into an area where ALL of the zombies are waiting for them, with M leading them saying they're ready to fight. The bonies start crashing in from the ceiling, and a brawl breaks out between them and the zombies. A couple of bonies almost attack R and Julie, but R fights them off. Some soldiers eventually come in and see the chaos, but M tells them to shoot the bonies, which they do. Grigio is alerted about this and is headed there.
R and Julie reach an area where they're cornered by bonies and have no place to run. There's a door that leads outside, but is high above a pool. Seeing no other way out, R takes Julie and they fall off the edge, landing in the water with R hitting the ground first. As Julie pulls him up, R appears to have almost completely transformed back to normal. His eyes become clearly blue again, and Julie kisses him. The moment is ruined, however, when a gunshot rings out, and R is hit in the chest. Grigio pulled the trigger and has other soldiers behind him. Julie defends R and shows her father that he really is alive as he is bleeding from the gunshot wound. Grigio and the other soldiers stand down, and he sends a message saying the situation has changed. R and Julie get out of the fountain, and she embraces her dad.
R is taken to get treatment, and he narrates the epilogue - the humans teamed up with the zombies to kill the rest of the bonies. Any bonies that were missed just wasted away. Through that act of teamwork, the zombies were able to continue changing back to normal. We even see M in the rain trying to get an umbrella open, when a young woman goes to help him. He covers her from the rain and introduces himself as Marcus.
R, now completely human again, is sitting on a bridge with Julie. She asks if he can remember his name now, but he can't. She tells him that he can make up a new one, but he likes just using R.
The film ends with the two of them watching the sun set, as the barrier between the city and the infected area is blown up and destroyed.