It starts with a nice Zombieland vibe with a narration and animations from our genial hero, Joel Dawson (Dylan O'Brien) who is living in a bunker with a group of friends. An asteroid has wiped out most life on Earth and caused reptiles and insects to grow to giant size and prey on the humans. In the bunker, all but Joel have paired up, leaving him lonely and with only kitchen duties to look forward to as he freezes up when attacked.
Joel decides that he needs to go and find a girl called Aimee (Jessica Henwick) who he was separated from when the humans first fled from the enlarged amphibians. So he sets off alone, armed only with his trusty bow and arrows. Along the way to the beach, where he knows Aimee is from her brief radio contact, he is rescued from a monster's nest by Clyde Dutton (Michael Rooker) and Minnow (Ariana Greenblatt) and makes a friend in a dog called Boy. There are monsters large and small and eventually he arrives at the beach only to find that Aimee and her colony have been 'rescued' by an Australian called Cap (Dan Ewing) ... but all is not as it seems.
As the film progresses, so much of it becomes simply charming. I loved the sequence with the active robot, giving Joel a glimpse of his parents again; the dog is incredible, acting so well on cue and stealing the show; the giant crab at the end, echoing what Joel had been told earlier ... and so on. Everything is worked for here, and the characters and the effects mesh seamlessly together to create a very believable narrative.