Malik Edward Furlong and his former girlfriend Gen Cerina Vincent, along with Gen's friend Barbie Amber Benson and their other friend Wes Callard Harris, head to a long-abandoned prospecting town in California, a place rumored to hide miles of tunnels that stretch toward the Mexican border less than a mile away. The quartet descends into the dark passages, where they are watched from a distance by a mysterious old man Steve Railsback, who touches a necklace and pours what appears to be blood onto the ground.
Inside the tunnels, the group stumbles upon two Mexican drug dealers, Jorge Alejandro Samaniego and Al Dean N. Arevalo, along with their teenage sidekick Zee Paul Cram, ferrying contraband across the border. The dealers force the four teens to tag along, and suddenly ghostly creatures appear, attacking from the shadows. The teens and the drug runners must flee together, scrambling toward the nearest hatch as the old man is seen silently summoning the spectral beings by stroking his necklace. One creature hurls a hook and chain, impaling Al in the process.
Barbie is ripped apart when she tries to stand on Jorge’s shoulders to reach another hatch, and Jorge loses most of his fingers in the chaos. Zee vanishes again, leaving the survivors to take shelter in a boarded-up shack. A creature pierces Wes through the neck, and Jorge is killed as the nightmarish encounter intensifies.
Malik and Gen uncover a room crowded with dozens of corpses that have piled up over the years. Zee returns and leads them to a hidden path. They manage a narrow escape back to the surface, only to realize they’ve misplaced the car keys and must seek a ride from a nearby house. The owner turns out to be the mysterious old man, though the teens do not yet know he is the mastermind behind the attacks.
On the drive, the old man offers them beer; Gen drinks and quickly loses consciousness. Malik awakens to find himself stabbed in the legs and knocked out with chloroform. They awaken back in the room of dead bodies, where they free themselves from the binding and discover a narrow tunnel leading to the old man’s house. There, they uncover a bedroom covered in newspaper clippings about Zee, a teenager murdered years earlier, with the old man named as a suspect. Zee reappears and reveals another passage beneath a bed; the old man soon follows, chasing them through the tunnel and summoning the creatures again.
A tense chase ensues, and the old man closes in on Gen, preparing to murder her by slow-necked stabbing, until Zee intervenes and tells him to stop. Gen fights back, seizes the necklace, and shatters it, at which point the creatures appear and kill the old man.
Gen cuts herself and bleeds onto a rock as she and Wes part ways for a final farewell. She and Malik push through to the surface and flag down a passing pickup for a ride to safety. In the dawn light, Malik muses aloud about the old man’s control over spirits and whether such power could be wielded by others, while Gen shrugs off the notion. The final image lingers on Wes and Barbie’s lifeless bodies, staring in silence at Malik and Gen from outside the motel window.
