PLOT
The destruction of an asteroid headed for Earth releases fallout, causing cold-blooded animals to mutate into large monsters and kill off most of humanity. During the evacuation of Fairfield, Joel Dawson is separated from his girlfriend Aimee but promises to find her, shortly before his parents are killed by monsters.
Seven years later, Joel lives in one of many bunkers underground called "colonies", where all other survivors have paired up romantically while fighting monsters and seeking supplies. Joel instead is relegated to the kitchen, as he freezes up in dangerous situations. After a giant ant breaches his colony, killing one of the survivors, Joel decides to set off on a quest to reunite with Aimee, so that he doesn't end up alone.
Passing through the suburbs, Joel is attacked by a giant toad-monster but is saved by a stray dog named "Boy", who follows Joel on his journey, warning him against poisonous berries and other dangers. Joel falls into a nest of worm-monsters called "Sand-Gobblers", when two survivors, Clyde Dutton and Minnow, rescue him. They are heading north to the mountains, where fewer monsters live due to the colder weather and higher elevation. They teach Joel some basic survival skills, and that not all monsters are hostile, demonstrating how "You can always tell in their eyes". They invite Joel to join them, but he insists he must find Aimee. As they part ways, Clyde gives Joel a grenade.
Continuing west, Boy becomes trapped by a giant centipede-monster. Joel freezes, but eventually saves Boy with his crossbow. Sheltering in an abandoned motel, they meet a robot named Mav1s. Before her battery dies, Mav1s powers his radio long enough for him to briefly contact Aimee. She tells him that other survivors have reached her colony, promising to lead them to safety. The next day Joel and Boy are attacked by a queen Sand-Gobbler. They hide, but Boy barks, giving away their position. Joel kills the queen with the grenade, and yells at Boy for putting them in danger, causing Boy to run away. After swimming across a pond, Joel is covered in poisonous leeches and hallucinates, but is rescued before he collapses.
Joel wakes to finally see Aimee. She leads a beach colony of elderly survivors who depend on her. He is introduced to the survivors, as well as Brooks "Cap" Wilkinson, a ship captain, and his crew, who had all recently arrived on a large yacht. As everyone celebrates their imminent departure, Aimee confesses to Joel that she is glad to see him, but that she has become a different person and is still mourning someone she was in love with. Joel decides to return to his colony, contacting them on the radio and learning that it has become unsafe there, and that they too must leave. Cap sends Joel some berries to eat, which he recognizes as poisonous. Realizing Cap is not to be trusted, he rushes to warn Aimee but is knocked unconscious.
Joel, Aimee, and the rest of her colony awaken tied up on the beach. Cap reveals that his group is going to raid the colony, and that their yacht is towed by a crab-monster controlled with an electrified chain. Cap sets the crab to feed on the colonists, but Joel and Aimee escape and are able to fight for their lives, and Boy returns to help. After a lengthy battle, Joel has the opportunity to shoot the crab, but he realizes it is not hostile by looking into its eyes. Joel instead shoots the electrified chain, freeing the crab, who leaves Joel and the others unharmed and instead sinks the yacht, devouring Cap and his crew.
Joel recommends to Aimee that she and her colony head north. They share a goodbye kiss, and Aimee promises she will find him. Joel treks all the way back to his colony, and they too decide to head to the mountains. On the radio, Joel inspires other colonies to take to the surface. As the colonies head north, Clyde and Minnow, already in the mountains, wonder if Joel will survive the next journey.
