Deep in the endless tunnels of a subterranean cave, an ancient species lies patiently in waiting for the creatures of the surface to step out of the sun in director Richard Pepin's horrific voyage into the black pit of the unknown. Rumor has it that a vast treasure lies hidden deep within an abandoned mine that sits just atop the vast system of caves, and a group of brave adventures are determined to claim the mythical prize for themselves, if it does in fact exist. But something far more sinister awaits the unsuspecting treasure hunters beneath the surface, and as their harrowing journey takes an unexpectedly terrifying turn, they soon discover that humankind is no longer at the top of the food chain.

Telly Paretta (Julianne Moore) believes that her son, Sam (Christopher Kovaleski), died fourteen months ago in a plane crash, but her husband Jim (Anthony Edwards) tells her that she's delusional and that they have never had a son and even her best friend Eliot (Jessica Hecht) doesn't appear to believe in Sam's existence despite her closeness to him. Dr. Munce (Gary Sinise) tells her that Sam was merely a figment of her imagination and is just imagining a life that might have been. He recommends that she be sent to a hospital, but she runs away and meets with a man named Ash (Dominic West) who she thinks is the father of a girl (Kathryn Faughnan) who was friends with her son and died in the same crash. At first he dismisses her, claiming he never had a daughter, and calls the police. After she is taken into custody, however, he remembers his daughter and rescues Telly. Together they escape and go into hiding, pursued by National Security agents. Telly and Ash capture an agent (Lee Tergesen), whom they threaten. The agent reluctantly reveals that he and other agents are merely helping "them" and that they do so to protect humankind. The roof of the house blows off and the agent, along with the roof, is sucked into the sky — presumably taken by "them"- and Telly and Ash flee. Eventually, Telly visits Dr. Munce again and he reveals that the disappearances are the work of "them," and that the government monitors their trials, all too aware that they have no power to stop "them" from doing whatever they want. Doctor Munce takes Telly to an airport and the dilapidated hangar of Quest Airlines, where he introduces her to an agent of “them” (Linus Roache). He tells the agent that “It’s over,” and to stop the experiment because it will only cause more harm. The agent replies, saying “It’s not over,” and then tells Telly that she has been a part of an experiment to test whether the bonds between mother and child can be diminished. In her case, her memories could not be erased. Telly refuses to deny her son's existence. The agent mentions that if he fails to erase her memory then he will look like a failure. The agent then subdues her and finally succeeds in erasing her memories of her son. During further questioning, though, a question about the name of her son triggers Telly's memory of her pregnancy, and all of her memories of Sam return. This causes the agent to be "abducted" himself, supposedly for his failure to erase her memory. This ends the experiment. Telly finds herself living a normal life, although she remembers everything that has happened. She reunites with Sam at a park. Also at the park is Ash, watching over his daughter. Like Sam, he has no memory of what has happened. Telly reintroduces herself, and the two sit and watch the kids play in the playground.