As the film begins we see Tricia removing old posters and replacing them with new ones. When she comes home, she finds that her younger sister Callie has arrived. She comes to live with her as the pressure mounts to finally declare Tricia's husband, who has been missing for seven years, 'dead in absentia.' Tricia is pregnant, but refuses to tell her sister who the father is. Tricia and Callie talk about Callie's road to recovery, as she has been addicted to drugs for quite some time.
Tricia is reluctant to declare her husband dead, always holding out hope, but Callie is practical and wants her to move on. Detective Mallory helps Tricia to fill out the paperwork and she and her sister visit a lawyer to get the ball rolling.
Callie likes to take a run and takes a tunnel which leads to the park. On one of her runs, she sees a man lying in the tunnel. He asks if she can see him and tells her his name is Walter. Callie promises to bring him some food later on, which she does, only to find the man has mysteriously vanished. She leave the food by the entrance of the tunnel, only to find that when she returns home, she has received a mysterious gift of keys, earrings and all kinds of small change. She tries to take it back to the tunnel, but the small pile keeps appearing in her house.
As Tricia comes to terms with her husband being declared death, she starts to move on with her life, finding a new apartment with Callies help, and cleaning out her husbands clothes. She even agrees to go on a date with Detective Mallory, who says to Callie he's been trying to get her to go out with him for some time. As the leave for their date, Tricia sees her dead husband standing on the street. As she has had numerous hallucinations of her husband before, she ignores him, only to be overwhelmed as Det. Mallory sees him as well and calls to Tricia to call an ambulance.
Daniel, Tricia's husband, is being examined at the hospital, but is unable or unwilling to provide any information about where he's been. When they take him home at the later time, the site of the tunnel causes him to pee himself. He later tells Callie she shouldn't have traded with it, but gives no information about what it is. Callie an Daniel hear strange sounds coming from the bathroom and when Callie goes to check it out, she's hit on the head and blacks out for a second. When she comes to, Daniel is being dragged out of the room and down the stairs. Callie follows and sees impressions of his hands and body in the ceiling above her. Daniel then falls out of the ceiling and is dragged outside. Callie tries to follow him only to see him being captured by an insect like creature, who drags him into the wall of the tunnel.
She tells all this to her sister and the two detectives, but they refuse to believe her and think she is under the influence of something. Later when Tricia and Callie are alone, Tricia searches Callie's room and finds a small box with drugs in them. When Tricia leaves, Callie goes online to research missing people in tunnels. She finds that there have been at least 20 people which have gone missing from the neighborhood over the last 50 years.
She shows her findings to Tricia, who has a hard time believing this could be something supernatural as reality is often as simple as it is. Later a detective comes to the door, asking if Callie could identify a body. It turns out Walter has turned up dead. Walters son, who has been trading with the force in the walls as well, goes insane when he sees his dad.
A while later Tricia and Callie are home again and have just turned out the lights to go to bed. They here a noise coming from the darkness of the living room and Tricia peers to see what's happening. Callie sees her being dragged out the door towards the tunnel. She tries her best to hold on to Tricia, but when their fingers slip, Tricia is being dragged into the darkness of the tunnel.
After relating this story to the detectives, who don't believe her, she goes home. She prints out all of her research and puts it in an enveloppe, addressed to det. Mallory. She then goes to the tunnel, demanding a trade. The walls around her come alive with screaming men and women and hands are being pressed into the concrete from the inside out. She tries to run out of the tunnel only to be snatched at the last minute, leaving only a sneaker behind.
The detectives find her research and her shoe, but don't give much credit to the story. They think up other courses of action which have Tricia and Callie leaving apart or together, to live off the grid. Detective Mallory is hanging missing posters in the neighborhood, when he sees a female figure standing at the entrance of the tunnel. When he approaches the tunnel however, there is no one to been seen. As he is walking away, we see a female figure looking through a haze at the disappearing detective, as a bug moves it's paws on her shoulder.