The catastrophic arrival of an asteroid on a collision course with Earth triggers a devastating chain reaction. The fallout transforms cold-blooded creatures into immense, lethal monsters, which rapidly decimate the human population, eliminating 95% within just a year. Amidst the chaos of Fairfield's evacuation, Joel Dawson, portrayed by Dylan O'Brien, gets separated from his girlfriend Aimee, played by Jessica Henwick. As tragedy strikes and his parents, Andrew Buchanan and Tandi Wright, succumb to the havoc—crushed in their car under the weight of a monster—Joel swears to find Aimee.
Fast forward seven years, Joel resides in an underground bunker, one of many colonies where survivors still cling to hope, but there’s a twist—everyone else has paired off romantically. Friends Ray and Karen, along with Ava and her partner Tim, have all formed bonds, while Joel remains isolated, often overwhelmed in treacherous situations. His monotonous days change dramatically when a massive ant breaches his sanctuary, claiming a friend's life and forcing Joel to embark on a courageous journey to reconnect with Aimee. With her colony situated 85 miles away, he must overcome perilous landscapes and monstrous foes.
During his perilous trek through abandoned neighborhoods, Joel encounters a giant toad-like creature but is saved by a stray dog he names Boy. This cutie not only accompanies him but also warns Joel about poisonous fruits and lurking dangers. However, when Joel inadvertently falls into a den infested with monstrous worms known as Sand-Gobblers, he is rescued by unexpected allies—Clyde Dutton, a seasoned survivor played by Michael Rooker, along with the brave Minnow, portrayed by Ariana Greenblatt. They reveal their plans to flee north to the mountains, where the weather is inhospitable for monsters. Clyde shares insights on survival, indicating, "You can always tell in their eyes," before gifting Joel a grenade for protection.
As Joel continues westward, his loyal companion Boy becomes ensnared by a colossal centipede creature. Initially paralyzed by fear, Joel comes through, utilizing his crossbow to free Boy. Later, they seek refuge in a rundown motel, where a dying robot named Mav1s provides a glimmer of hope by briefly restoring Joel's radio to contact Aimee. Unfortunately, others have reached Aimee's colony, jeopardizing their safety.
Their next day plunges them into danger as a queen Sand-Gobbler attacks. While they attempt to hide, Boy's barking gives them away, and Joel makes a bold move, using the grenade to eliminate the threat. In the ensuing chaos, Joel scolds Boy, who, hurt by the reprimand, runs away. After a chance encounter involving hazardous leeches and hallucinations in a pond, Joel wakes to a heartwarming reunion with Aimee, who is caring for a beach colony made up of elderly survivors. Among them is Cap, played by Dan Ewing, along with Dana and Rocko. As celebrations unfold, Aimee expresses her mixed feelings—she firmly declares, “I’m glad to see you, but I've changed.”
Realizing he never truly considered Aimee's desires, Joel resolves to return to his own colony. He contacts them only to learn of the new dangers lurking there. When he sends a warning, Cap's true intentions emerge—he and his crew aim to exploit Aimee's colony for supplies, and Joel finds himself imprisoned alongside the others. However, an audacious escape unfolds. Together, Aimee and Joel fight back, with Boy returning to lend a paw.
In a climactic moment, Joel chooses not to destroy Cap's crab-monster henchman after recognizing its non-hostility in its eyes, opting instead to sever its chain. The creature defies expectations by turning against Cap, resulting in the destruction of their stolen yacht.
Amidst the aftermath, Joel strategizes to send Aimee and her group northward for safety. Their farewell is tender, marked by a meaningful kiss and a promise to reunite. Joel then sets forth on his own arduous journey back to the colony, motivating them to seek refuge in the mountains. Meanwhile, Clyde and Minnow wonder if Joel's indomitable spirit will see him through the dangers that lie ahead.
