The film opens with Jessica Martin, a high school biology teacher, talking to her son Ricky, while escorting him to the school bus. After she returns home, mysterious assailants enter her home through the back door, kill her house maid, kidnap her and confine her in the attic of their safe house. Ethan, the gang leader, smashes the only telephone in the attic to prevent her from contacting anyone. She has no idea who the kidnappers are or what they want. She pieces together the broken phone to randomly make a connection. She finally reaches the cell phone of Ryan who has just been dumped by his girlfriend for being too irresponsible. He takes it as a joke, but Jessica persuades him to go to the police. At the police station, Officer Mooney tells him to go to the detectives on the fourth floor. He begins to lose the signal in the stairwell, and must turn back to avoid losing the connection.
Meanwhile, Ethan returns to the safehouse and asks Jessica about something she doesn't know. When Jessica tells him that she doesn't know, he tells her that he is going to get her son. Ryan, who overhears them, is now convinced that the kidnapping is real. After Ethan leaves, she tells Ryan to get to her son's school before they do. Unfortunately, he is too late and her son, Ricky, is kidnapped and he quickly chases them by stealing a car owned by a school security officer. Because his cell phone's battery is dying, he drives to a shop for a charger. After being repeatedly redirected from counter to counter, he uses a gun from the security vehicle to hold up the store at gun-point to obtain the device.
Sgt. Mooney, meanwhile, decides to check on the kidnapping claim that he received. He uses the Department of Motor Vehicles records to find the address of Jessica, but when he comes to her house, a woman meets him, telling him that she is Jessica and that everything is fine. Believing it to be a false alarm, he leaves. It is revealed that the woman is Dana Bayback, an accomplice of the kidnappers.
Ethan returns to the safe house and asks Jessica for the location of a place called "The Left Field", where her husband, Craig, was. Ethan then shows that he has imprisoned Ricky in the garage and threatens to kill him if Jessica does not tell him what this this information means. She tells him that Left Field is a bar in the LAX airport. As he leaves, she tells Ryan that they have gone to get her husband. Suddenly a cross-connection between phone lines threatens the carrier signal. A lawyer talking to his mother breaks in. Ryan is able to find the lawyer and steal his car and phone, since his phone was cut off from Jessica's and the security car was just destroyed by an oncoming vehicle. She tells him to head for LAX and find her husband. At the airport, he tries to stop the kidnappers by planting the security firearm in the bag of one of them as they go through security, but when they are apprehended, they reveal that they are cops. This causes Ryan and Jessica to realize that Ethan and his gang are dirty police officers. Ryan then finds a man that apparently matches Jessica's description of her husband Craig, but this mistake in identity permits the kidnappers to apprehend the real Craig.
On exiting the airport, he finds that the lawyer's car has been impounded. Meanwhile, a series of bizarre incidents have snaked their way into the news, including the one about a gunman who took a mobile charger and overpaid for it and an interview with the lawyer who states that his car was stolen by a man claiming it was to rescue a woman named Jessica Martin. Mooney sees the news and identifies Ryan. He calls Jessica's home and gets the voice mail, but this time notices that Jessica's voice on the answering machine is very different from the accented voice of the woman who claimed to be her.
Craig is brought into the attic and forced to reveal the location of a videotape. He tells them that it is at Centurion Bank in his safe deposit box, but that they need him to retrieve it. Before they leave, Jessica acts a little fanatical to whisper in secret to Craig that he will have help at the bank. Ethan and his friends Dimitri and Deason go with Craig while another kidnapper stays on guard. Ryan also reaches the bank. The kidnappers retrieve the video-camera, but Ryan knocks down Dimitri, takes the camera, and flees to roof alone after failing to take Craig with him. However, he accidentally drops the lawyer's cell phone off the roof, smashing it to pieces. He manages to escape in a taxi-cab, telling the driver to go to the LAPD Auto Impound. In the cab, he checks out the camera. He sees that when Craig was taking some video footage of houses for his realtor job, Craig accidentally shot footage of Los Angeles Police Department Detectives Ethan, Mad Dog, Dimitri, Bayback, Deason, and Jack Tanner, a friend of Mooney, robbing and murdering drug dealers (which is a good thing that Ryan left the police station when he did, as the detective that Mooney was sending him to was in fact Tanner). After getting off at the impound lot and sneaking in while the receptionist is distracted by the lawyer trying to get his car back without paying a fee, Ryan steals the lawyer's car again and gets back his own cellphone, relieved to find that Jessica's call is still on hold.
Mooney returns to the Martin residence, where Bayback shoots at him, injuring him. He retaliates and kills her, but learns to his dismay that she was a cop, too. Meanwhile, Mad Dog stumbles upon the phone line Jessica is using from the downstairs phone, and Jessica is forced to kill him by cutting his brachial artery before he could kill her causing him to bleed out in seconds. She attempts to escape with her son, but Ethan returns with Craig as a hostage and stops her. He is angry with her, wanting to know who that kid at the bank was. Before Ethan can do anything, Ryan uses his cell phone's memory to contact Ethan and makes a deal directly over the phone: the video tape in exchange for the Martin family. Upon learning of the meeting, Tanner convinces Mooney to delay his trip to the hospital for stitches, so that he can identify Ryan, who Mooney still thinks of as a prime suspect.
The deal goes down at the Santa Monica Pier. Ryan tries to handle it his way in disguise, but his ex-girlfriend accidentally exposes him, after which Mooney is able to finger him. While Tanner sends Dimitri to help Mooney get needed medical attention, he takes Ryan to Ethan. Ethan destroys the video recording and Tanner radios the order to kill the Martins, although Deason in the van suggests to wait until they get to the safe house. However, Mooney overhears the radio transmission from Dimitri's radio and he realizes that Tanner is one of the kidnappers. Ryan escapes following a distraction from his friend Chad, while Dmitri attempts to kill Mooney, but Mooney overpowers and handcuffs him. Tanner and Ethan confront Ryan in a boathouse, where Ryan knocks out Tanner with a surfboard, but Ethan beats him up with his superior fighting skills until Mooney intervenes. After a brief cat and mouse game, Ryan, wounded, notices that Ethan has circled behind Mooney, and helps Mooney by calling Ethan's cell phone (revealing that Ryan's phone somehow was not water damaged after jumping into the river). The ring of the cell betrays Ethan's hiding place, and Mooney promptly shoots him dead. As Ethan falls, he looks dumbfoundedly at Ryan ... and then at Ryan's cell phone, the "weapon" that got him killed just before passing away.
While this was going on, Jessica manages to strangle Deason with her handcuff chain from the rear of their van, then free her husband and son. But Deason was merely stunned, and aims his gun at them. Then Ryan suddenly intervenes and smashes him around till he's unconscious.
While Ryan and Mooney are being treated by medics, Tanner is also exposed, because Ryan had made a copy of the videocam recording onto his cell phone, and the Martin family is set free. Jessica finally gets to meet the man who has risked his life saving her and her family. Ryan jokingly requests for her is never to call him again, and they laugh.