In a flashback to "Day One," Lee, Evelyn, Marcus, Regan, and young Beau Abbott attend Marcus's Little League baseball game alongside family friend Emmett. Mid-game, a flaming meteor streaks across the sky. Within minutes, blind extraterrestrial predators arrive in town, tearing through buildings and hunting down residents by their screams. Lee, Regan, and Evelyn survive the daylight attack and retreat to safety.
In the present on Day 474, following Lee's death and the destruction of their farm, Evelyn leads Regan, Marcus, and her newborn infant past the perimeter of their sand path to search for other survivors. Crossing an abandoned rail yard, Marcus steps into a steel bear trap, which snaps shut around his leg and shatters bone. Struggling to stifle Marcus's agonizing screams, Evelyn uses her shotgun and Regan's feedback-broadcasting amplifier to kill a creature that rushes them.
The family is ushered into an abandoned industrial steel foundry by a cloaked survivor who reveals himself as Emmett. Emmett, having lost his wife and children to the invasion, provides them temporary shelter inside an airtight underground furnace vault, but warns them they cannot stay due to limited rations. Marcus discovers that a local radio station has been playing Bobby Darin's song "Beyond the Sea" on loop for months. Regan deduces that the song is an audio clue pointing to an offshore island community where survivors live safely.
When Marcus cautions that their mother will never agree to the journey, Regan sneaks out at dawn with her makeshift amplifier and map to find the broadcast source alone. Evelyn wakes, discovers Regan gone, and begs Emmett to track down her daughter. Emmett catches up to Regan inside an abandoned passenger train car just as she is cornered by a creature, rescuing her in the nick of time. Moved by her bravery and remembering Lee's spirit, Emmett agrees to help her reach the marina.
Back at the foundry, Evelyn leaves Marcus and the baby in the bunker to scavenge medical supplies and oxygen tanks in town. Marcus leaves the bunker to explore the foundry, discovering the dead body of Emmett's wife. Startled, he knocks over equipment, drawing an alien into the building. Marcus rushes back to the underground vault, locking himself and the baby inside as the air runs out.
At the marina docks, Emmett and Regan are ambushed by a feral group of human scavengers. Emmett intentionally creates noise to draw creatures to the pier, causing the beasts to wipe out the attackers. During the chaos, Emmett realizes the monsters cannot swim, and he and Regan paddle out in a small motorboat to safety, arriving at a peaceful island commune led by a man who explains the military evacuated people to the island when they realized water creates a barrier.
The peace is shattered the following morning when an empty boat carried by ocean currents drifts onto the island shore with a creature aboard. The alien attacks the colony, killing the community leader and chasing Emmett and Regan into the local radio station. At the same time, Evelyn returns to the foundry, where she, Marcus, and the baby are pinned down by the second alien.
