The story begins at a mining operation where workers discover a terrifying problem in the c-shaft. Demonic subterranean monsters erupt to the surface, slaughtering everyone in their path. This phenomenon occurs globally wherever deep holes have been drilled. Despite military efforts, the creatures prove unstoppable until they suddenly retreat after 24 hours, an event later dubbed the Day of Reckoning. In response, world governments establish monitoring stations at deep-core fissures to seal them off.
Fifteen years later, the world lives in fear of the creatures' return. Patterns have emerged: attacks are preceded by masses of dead fish washing ashore, bird migrations, and a lunar eclipse. As another eclipse approaches, tensions rise. We are introduced to David Sheppard, a chief of maintenance, and his ex-wife Laura, a professor who is being pursued by her colleague, Milton. Their son, Tyler, is a high school student dating a girl named Maddy. Meanwhile, Garrett Jensen, a corporal in homeland security stationed at a fissure, senses that another Day of Reckoning is imminent.
The chaos begins when David's uncle Ted warns him to reach his underground bunker in Barstow by noon. As monsters emerge—including dog-like creatures that kill soldiers at Garrett's post—David struggles through traffic to find Tyler. They are eventually picked up by Laura and Milton. After being turned away from an overcrowded shelter in Topanga, the group flees toward Barstow. During their journey, a flying creature snatches Milton, leaving the others to navigate a drainage culvert. David reveals a critical weakness: the demons can be killed by salt and cold.
Upon reaching the bunker, they find Ted is being held at gunpoint by Garrett, who had been let in by Ted's wife, Stella. Garrett theorizes that the monsters feed on human souls. After a tense confrontation and a battle with a snake-demon—which Maddy successfully kills—the group manages to subdue Garrett.
As the bunker is besieged by a bull-demon, the survivors use a back tunnel to escape toward a security gate. In a tragic turn, Stella sacrifices herself to the demons to allow the others to escape in Garrett's car. Once inside the secure facility, David discovers stores of saltwater. He courageously reroutes the saline into the fire suppression system, and Tyler triggers the alarm, raining saltwater down on the monsters and melting them into embers.
In the final moments, as the demons return to the underground, a delusional Garrett shoots Tyler, claiming it is the Devil's Day. However, Garrett is promptly swallowed whole by a massive centipede demon. By crafting a cold salt bomb, the family destroys the remaining creature and escapes the gate, surviving until the next event.
