Jordan (Halle Berry), a Los Angeles 911 operator, takes a call from Leah Templeton (Evie Thompson), a pretty blonde teen. A man has broken into Leah's house. Jordan dispatches officers to her house, and Leah hides; the man seems to be leaving. Then the call cuts out, and, without thinking, Jordan hits the redial button. The intruder pauses when he hears the phone ring once and then stop - Leah answered it, and now she hears him returning to the room where she hides. The man finds Leah, and he takes the phone. Jordan begs him not to hurt Leah, but he tells her, "It's already done." Leah screams, and the call ends.
In the wake of Leah's disappearance and the subsequent discovery of her mutilated body, Jordan suffers a breakdown. Though her colleagues tell her to move on, she no longer trusts her judgment. Six months later she has become a teacher at the 911 operator headquarters.
Casey (Abigail Breslin), a carefree blonde teen, leaves a mall. She has two cell phones: her own smart phone and the prepaid cell phone her best friend Autumn left behind when she ditched her at the food court. A maroon sedan almost hits Casey in the parking garage, and she drops her smart phone. The driver jumps out and apologizes, but within seconds he has wrapped his hands around Casey's mouth and body. He suffocates her until she passes out.
Jordan leads her latest class through the "hive," the room where the operators take calls. She stops to introduce the students to a new 911 operator, but the field trip takes a frightening turn when the new operator answers a call from Casey. The latter found her friend's prepaid cell phone, and she's calling from the car trunk where her abductor has stashed her. Jordan quickly takes over, and when Casey cries she will die, Jordan's old anxieties rear their head. She quickly regains composure and calms Casey down so she can learn more information about Casey's location - prepaid cell phones lack GPS chips, so the police and Jordan have no way of knowing where the moving car is. Dozens of police officers, including Jordans old flame Paul, mobilize to find Casey.
By talking to Casey, Jordan figures out: the car is on a highway. It is an older model, the driver was a white man in his thirties, and there are several cans of paint in the trunk. Casey finds a shovel, too, and panics until Jordan tells her to kick out one of the taillights. After knocking out the taillight, Casey sticks her hand through the hole and attracts the attention of another driver. The woman calls 911, and she gives the operator her location and the cars plates (which turn out to have been switched). Unfortunately, the woman pulls forward to see the driver, and when he notices, he swiftly exits the freeway.
Next, Jordan tells Casey to open the paint and pour it out of the taillight hole, thereby leaving a trail. Alan Denado (Michael Imperioli) notices the mess and alerts the driver, and the drivers shifty behavior arouses his suspicions. Caseys kidnapper pulls into an empty parking lot and flips out at her when he sees the mess she has caused. He decides to drug Casey with chloroform, and while hes bent over in the trunk, Alan Denado appears and asks if he needs help. The kidnapper behaves even more weirdly, and Denado pretends to leave, but in actuality he tries to call 911. The kidnapper attacks Denado, smashes the phone, and beats the would-be rescuer with a shovel. His bottle of chloroform breaks in the process he leaves it in the parking lot and he freaks out even more before stealing Denados car.
Casey wakes up in the trunk and freaks out when she sees Denado. Then Denado awakens and begins to scream. Casey begs him to stop, but the kidnapper hears the commotion, and this time when he pulls over he makes sure to kill Denado. Jordan, still on the line, instructs Casey to find Denados wallet and identification. The officers find the kidnappers abandoned car. Paul notices the chloroform bottle and has it dusted for fingerprints. Back in the trunk, Casey once again believes she will die, and she tells her mom that she loves her so much, knowing the operators record all of their calls. Jordan tells her to fight. An Amber Alert has gone out about Casey.
Denados car is low on gas, so the kidnapper pulls into a gas station. An invigorated Casey finds out she can crawl into the cars interior through the middle backseat, and she catches the attention of the gas station owner. When the man tries to rescue Casey, the kidnapper douses him in gasoline and lights him on fire. He opens the trunk, punches Casey unconscious, and drives away.
Paul has an I.D. on the kidnapper: Michael Foster (Michael Eklund), a family man and medical technician. He and his partner go to Fosters house and interrogate his wife about where Foster might be. Paul finds a shrine dedicated to Fosters beautiful blonde sister, who died some years ago. Eventually, he discovers that Foster owns a cabin in the San Clarita Hills. The squad mobilizes.
Foster finally parks. When he opens the trunk, he finds Casey holding the cell phone. He panics, and Jordan begs him not to hurt Casey, calling him by name. Foster tells her, Its already done. Jordan realizes Foster killed Leah Templeton, too. Foster smashes the phone, and after Jordan loses contact with Casey, her boss tells her to go home. Paul and his team swarm Fosters cabin but find no trace of the kidnapper or his victim.
Somewhere else, Foster has strapped Casey into a wheelchair and wheels her into his laboratory. He forces her to inhale sedatives, but they wear off, and Casey manages to slip her bindings. She runs down a hallway and into a room, but she recoils in horror when she sees whats inside, and Foster recaptures her. Back at the hive, Jordan replays different clips of her conversation with Casey. She replays the part right before Foster destroyed the phone, and she fixates on a clanging sound before losing her cool and leaving the hive. She drives to Fosters cabin the police have left and searches the rooms. She finds pictures of Foster and his sister, who seems to hug and cuddle in a lot of the photos. Several snapshots indicate that Fosters sister died of cancer, and the last shot shows a young Foster kissing his hairless, emaciated sister on the mouth.
Jordan steps outside. She hears a clanging noise like the one from the recording its a lock banging against a flagpole. Jordan finds a hatch in the ground and discovers Fosters subterranean lair. Jordan creeps around the lair and finds the same room in which Casey tried to hide its a replica of Fosters sisters room, but the pink bed is smeared with blood. Jordan hides when Foster enters the room, and shes horrified to discover that what she thought were wigs are actually the scalps of Fosters victims. Its obvious he has been killing girls to reenact his twisted fantasies of his sister.
Casey is strapped to an operating table. Foster begins to scalp her, but Jordan interrupts him by smashing him in the head. She tries to free Casey, but Foster recovers. He starts to drown Jordan in a basin of water. Casey frees herself and slices open Fosters face. The women run out of the lair, but Foster follows them. Casey eventually stabs Foster in the back and knocks him back into his lair. Jordan almost calls 911, but Casey stops her.
When Foster regains consciousness, he is chained to the wheelchair. Jordan and Casey tell him their version of Caseys rescue: Jordan will find her wandering in the woods, and he will disappear forever. They leave Foster in his dungeon, immune to his desperate cries for help.