It's a representation of what the first month would be like in a zombie apocalypse (trying to find some one else who is alive, saving enough food for each day, counting the days, trying to find a way to communicate to others etc via cell phone with no bars). Its basically that, barely any talking. I could literally give you the whole movie synopsis in less than a paragraph:
boy in apartment: apocalypse breaks out, he doesn't have much food, finds girl across the street and communicates and sends food to her via rope: both decide to go and find more food together: finds another survivor to only use the girl as food for his zombiefied wife~ foe gets killed, both kids hear helicopter and go to the roof fighting off zombies~ barely makes it on the helicopter alive but does!
It was good enough to maybe watch again if I was with a friend