Three years ago, the world was upended when terrifying apex predators known as Reapers emerged from underground sinkholes and decimated humanity, annihilating 95% of the population. Now, the few remaining survivors cling to life atop mountain communities, perched at elevations above 8,000 feet—altitudes these creatures inexplicably cannot cross. In one such settlement, single father Will lives with his asthmatic son Hunter, whose fragile lungs require oxygen filters that are running dangerously low.

Haunted by the memory of his wife Tara—killed by Reapers during an expedition with scientist Nina—Will finds himself pushed beyond survival when Hunter’s condition deteriorates. Hunter’s yearning for connection, highlighted by his venturing below “the line” to glimpse other survivors, underscores the emotional stakes of their isolation. Will realizes that to save his son, he must risk everything and descend below the safety of the highlands.

Despite Nina’s grave warnings about the dangers below, Will persuades her to accompany him along with Katie, his late wife’s friend, on a perilous journey to Boulder. Their path leads them through abandoned infrastructure and into dark tunnels, confronting them with the deadly reality of the Reapers. Will and Nina cling to the hope of finding oxygen filters in the hospital—filters that could keep Hunter alive—and maybe even find a way to fight back.

Nina, whose family was also lost to the Reapers, reveals a bold theory: the creatures’ impenetrable armored scales might be vulnerable to a bullet coated with a magnesium-based compound that could trigger internal combustion. Braving the terror of the lower world, the group presses on, and in a tense confrontation at the lab, Nina successfully tests her theory—triggering a Reaper’s catastrophic destruction.

But revelation brings new danger. As Will flees through treacherous terrain to return to Hunter, he is pursued by multiple Reapers—only to be saved at the last moment by Nina, who again wields her Co/Mg‑laced bullets to devastating effect. In the aftermath, they realize these creatures are not biological beasts at all, but highly advanced alien machines sent to exterminate humanity.

Reunited at Lost Gulch Refuge, Will and Nina signal triumph—and defiance—by raising a pirate flag, inspiring other enclaves to resume radio contact and prepare armed resistance. As they share a quiet moment gazing skyward, three greenish meteorites streak across the horizon—hinting that more Reapers, or their alien creators, are on the way.