Bryan (Liam Neeson) is a retired Government Agent. He is at an electronics store and is carefully choosing a karaoke device for his beloved 17-year-old daughter Kim's (Maggie Grace) birthday. As Bryan arrives at a luxury mansion, Kim's mother Lenore (Famke Janssen) is very cold toward him, criticizing his choice of a gift and that he has even showed in the first place. The daughter is extremely pleased with her gift; however, she is soon distracted by her wealthy stepfather Stuart's (Xander Berkeley) gift of a large horse. Later, Bryan is alone in his small modest apartment. Three friends, all also retired agents, drop by for poker, steaks and beer. One of them, Sam (Leland Orser) invites Bryan to make some quick cash working security with them by escorting a female pop star to her nearby concert. After her vocal warm-up Bryan asks the pop star, Sheerah (Holly Valance) for advice to give his daughter who also wishes to be a singer. Sheerah answers, "Tell her to pick another career." Bryan gets a phone call from Kim inviting him to lunch. At the concert, things get anxious for a moment when security is distracted by a mob of fans breaking through. Bryan escorts Sheerah away from the crowd, but is confronted with a knife-wielding assailant that Bryan quickly takes down. As a "Thank You," Sheerah offers to help Bryan's daughter with a vocal coach and a manager if she has any natural talent.
Bryan is very excited and plans to let Kim know about Sheerah's offer; however, Lenore comes to the lunch also and it becomes clear he was invited only to sign the parental-consent form Kim needs to take a vacation to Paris with her friend Amanda (Katie Cassidy). Bryan is reluctant to agree, knowing the dangers of the wrong parts of Paris and feeling Kim is too young, but Kim explains she'll be staying with Amanda's cousin's and spend most of her time in museums. Bryan says he will consider the matter, but Kim takes it as a "no" and becomes very upset. Lenore tears into Bryan for being too cautious. She wants their daughter to experience life. The next day, Bryan shows up at Stuart's mansion with the signed consent, a pre-paid international cell phone, and some conditions: 1) Bryan must take Kim to the airport; 2) Kim must call Bryan when she lands in Paris; and 3) Kim must call Bryan every night to tell him the address and phone number of any new place at which she will be sleeping. At the airport, as Bryan is retrieving Kim's luggage from the car, he sees a map of Europe with several cities circled in a pocket of Kim's luggage. He confronts Lenore who admits that the girls are actually going to follow the rock band U2 on their European tour, not just stay in Paris. Bryan is angry, but it is too late to do anything about it.
In Paris, while standing in the line for a taxi at the airport, the two girls meet a friendly, handsome young Parisian named Peter (Nicolas Giraud). He suggests they all share a taxi for the expensive ride downtown. Amanda immediately accepts and Kim reluctantly agrees. While they wait, they have Peter take their picture with Kim's phone. Appearing chivalrous, Peter has the taxi drop the girls off first. He invites them to a party that very night and Amanda eagerly accepts. As the taxi pulls away, Peter makes a quick phone call-he simply says "Two girls, about 18" and the address of the apartment. Upstairs, Kim inquires where the cousins are at. Amanda breezily announces the cousins are on vacation in Spain and giggly announces as she jumps on the couch that she plans to have sex with Peter later on.
Back in the US, a concerned Bryan checks the arrival information and learns that Kim's plane has landed. Bryan attempts to call Kim's phone; however, Amanda has cranked the music in the apartment to such a volume that neither girl hears Kim's phone ring. Finally, Bryan gets hold of Kim. Kim walks around to the other side of the loft (shaped like a square with a courtyard in between) to hear her father better. While they talk, Kim can see Amanda through the windows across the courtyard. As she talks to her dad, she suddenly spots two men grab Amanda and carry her away from view while Amanda kicks and fights. Panicked, Kim tells Bryan what she just saw. Bryan immediately takes control and orders her to go into the nearest bedroom and hide under the bed. Guiding her with a careful series of questions, he learns that Kim and Amanda shared a cab with a man named Peter. Kim thinks she saw three or four man take Amanda. "Now," he tells her, "the next part is very important. They are going to take you." Bryan tells her that she must leave her phone on, and she will have five to ten seconds to scream out any description of them she can. Bryan hooks up a tape recorder to his phone. Two men enter the bedroom and are puzzled they haven't found the second girl. They converse in a strange foreign language. Bryan tells Kim to hold the phone towards the men so he can hear their conversation. The two men appear to have left the room and Kim is relieved and tells her dad they left, when suddenly Kim is snatched from under the bed. Following Bryan's instructions, she screams out, "Beard! Six Feet! Tattoo, right hand, moon and star!" Bryan hears her scream for a few more seconds, then, after a moment of quiet, Bryan hears heavy breathing. Bryan tells the kidnapper that if he lets his daughter go immediately, he won't pursue him. If the man doesn't let Kim go, then he'll hunt him, citing his skills as a covert agent. The kidnapper, possessing a thick accent responds, "Good luck,".
Bryan sends the taped conversation to Sam, his ex-agency friend from the concert. Bryan then storms over to Stuart's, his wife's second husband, and confronts him about possible business enemies of Stuart arranging Kim's kidnapping. Stuart and Lenore are surprised about how much of Stuart's business Bryan knows. Bryan is satisfied that Stuart's business affairs are not the source of the problem, and he tells Stuart he needs a private jet to Paris "an hour ago." Bryan searches Kim's room and all her personal possessions for possible clues. Meanwhile, he is on speaker phone with Sam so Lenore can hear. Sam provides a detailed report that identifies the language as Albanian and he even narrows the accent down to a specific town in Albania. Their leader's name is Marko and he is a very powerful figure. Sam tells him that the Albanians kidnap girls for prostitution and Bryan has, at most, 96 hours (four days) before Kim will disappear forever.
On the private jet, Bryan plays the recording of Kim's abduction over and over again, committing every sound to memory. In Paris, Bryan first gains access to Amanda's friends' apartment. He visualizes the abduction from the clues left behind at the scene and finds and recovers the memory card from Kim's smashed cellphone which is adjacent to the bed under which Kim tried to hide. At a do-it-yourself photo kiosk, Bryan finds the picture Peter took at the airport and zooms in on a reflection of Peter in the photo. At the airport, Peter is using the same "share a taxi" line with a Swedish blonde. Bryan assaults Peter to find out about the girls from yesterday; however a large man, Peter's accomplice, tries to stop Bryan. As Bryan fights off the large man, Peter runs away. Bryan disposes of Peter's accomplice and gives chase stealing the taxi cab. Peter runs into traffic and Bryan jumps out of the taxi and chases Peter on foot. Peter then jumps over the overpass and seemingly lands safely when a truck hits and kills him.
Bryan then meets with an old acquaintance named Jean-Claude, who is with French Intelligence, to ask for help. The Frenchman tells Bryan where the Albanian-pimped hookers are located, but warns Bryan to stay out of it and to not cause any problems. As Bryan leaves, Jean-Claude sets up a tail to follow him. After getting a rental car, Bryan meets with an Albanian interpreter whom he's hired at the hooker area, first telling him to wait in the car after they drive to the Pigalle area of Paris. Bryan then deliberately wastes a prostitute's time so her pimp will come over to rough up the seemingly mild-mannered Bryan and demand some money. Bryan then slyly plants a listening device on the pimp. Back in the car, Bryan has the interpreter listen to the audio feed from the bug and translate the Albanian conversation. They talk about having trouble with some new merchandise at a nearby construction site. Bryan excuses the interpreter, but not before getting the Albanian-English dictionary he had asked him to bring.
At the construction site, there is a long line of men waiting to get into a trailer. Bryan acts like a customer and joins the line. When Bryan reaches the head of the line, he hands the "doorman" his money and receives a scrap of paper with a single digit on it. Inside the trailer ropes have been strung with blankets draped over them to form 'rooms' on either side of a center aisle. As Bryan walks down the aisle, he looks into each 'room.' Bryan enters one room and finds a cot with a drugged woman lying on it. She isn't Kim. She beckons him to come to her, but, instead, he starts pulling more blankets aside to check the other rooms. Since each room has a customer, this invasion of privacy starts a ruckus. Bryan has to deal with a few bouncers as he makes his way along the length of the trailer. In the last room, Bryan sees his daughter's jacket and a drugged woman with hair like Kim's. Rushing in, he finds it isn't Kim. Bryan tries to question this woman, but she is so drugged she can't talk. Bryan carries her to a nearby Jeep, hot-wires the ignition, and, after a wild chase, makes his escape from a swarm of angry Albanians. He drives to a quiet out-of-the-way hotel and rents two rooms from the owner (another old acquaintance). In one of the rooms, Bryan rigs up an IV drip and injects the girl with Narcan, beginning an ultra-rapid detox of her system.
Later, Jean-Claude demands a meeting with Bryan out in the open by the river. When Jean-Claude arrives, his cell phone rings. It is Bryan saying he doesn't trust meeting Jean-Claude (until he gets Kim back). Jean-Claude says his boss hit the roof over the construction site violence. While the boss wanted Bryan arrested, Jean-Claude has convinced him to authorize instead a ticket to send Bryan back to America, "for previous services." Bryan refuses. Meanwhile, Jean-Claude's men have been triangulating the source of Bryan's phone call and a squad of men burst onto a nearby rooftop. There they find a cell phone rigged with a walkie-talkie. Bryan is actually on another roof nearby. Bryan walks away easily.
The girl comes back to consciousness and tells Bryan she got the jacket from another girl. All she can remember is it was at a house with red doors on the rue Paradis. Bryan finds the house with the red doors, walks in bold and confident, and presents Jean-Claude's business card as his own. Bryan is led past several thugs to a dirty kitchen. Four Albanians are seated around a table. Bryan tries to ascertain who is the leader, but they all claim to be Marko and all from the town that Bryan's language expert had identified. Bryan tells them that their former police contact, Mr. Macon, had been transferred and now he (Bryan as Jean-Claude) will be collecting the bribes, the rate for which has just gone up. After some hard bargaining, a deal is struck and Bryan thinks he has identified the figure from Kim's phone. However, before he leaves, Bryan stops and says a friend gave him a note in Albanian. He hands it to the apparent leader and asks him to translate it. The leader (the real Marko) responds "Good Luck." Bryan recognizes Marko's voice immediately and says to him, "You don't remember me do you? ... I told you I was going to find you," and, suddenly Bryan is shooting and fighting the house full of armed men. When the smoke clears, Bryan has killed all of them but Marko. Bryan searches for Kim in all the upper bedrooms of the building. He finds several women handcuffed to beds and Amanda's dead body (apparently due to a drug overdose). Bryan tortures Marko with electricity. After trying to tough it out, Marko tells Bryan Kim was sold to a man named Saint Clair. Bryan then turns the power back on and leaves Marko screaming in agony, presumably to die.
Jean-Claude comes home for supper to discover Bryan is visiting like an old friend. Jean-Claude's wife, Isabelle, tells Jean-Claude to put the two kids to bed. As he does, he recovers a pistol from the lavatory and tucks it into the back of his waistband. At the dining room table, Bryan casually speaks about moving to Paris and drops the address of the Albanian pimp-house and Macon's name. Isabelle naively says that she herself knows Macon. Bryan then confronts Jean-Claude on selling protection and anonymity to the Albanians and demands to know who Saint Clair is. Jean-Claude pulls out his pistol and aims it at Bryan, angrily detailing the gap between his pay and his expenses. Bryan then tosses a handful of bullets onto the dining room table and tells Jean-Claude that he has been behind a desk too long if he can't tell the difference in weight between a loaded gun and an unloaded one. Bryan pulls his own pistol and shoots Isabelle in the arm, saying, it's a flesh wound but that his next shot will kill her unless Jean-Claude provides him with information on Saint Clair. Jean-Claude then goes into his office and uses his computer to access police files that identify and locate Saint Clair. Bryan then tells Jean-Claude to tell his wife he is sorry for shooting her and he then pistol whips the Frenchman to the floor.
Again posing as Jean-Claude, Bryan gains access to a party Saint Clair is hosting. Bryan discovers Saint Clair is hosting a much darker "party" on a lower level. Bryan assumes the guise of a waiter and gains access to a darkened room facing a wall of glass, one of a series of the same types of rooms that encircle an open stage. On the stage are a young, drugged, scantily clad young woman and a handler. In each darkened cubicle, a bidder presses a red button when they wish to bid. The auction ends and the girl is led away. The auctioneer announces the last and best woman of the evening's auction: an American, pure virgin; the girl is Kim. Bryan pulls a gun on the buyer in this darkened room and forces him to outbid the others to purchase her. The bidding goes to 500,000, but, upon leaving the room with the buyer still at gunpoint, Bryan is knocked unconscious from behind. He awakens suspended from a pipe just below the ceiling. He admits to the Englishman, Saint Clair, that he is the last girl's father. Saint Clair says, "too bad. It's business and she is sold already." He then orders his henchmen to execute Bryan quietly and leaves the room. Despite being shackled to the overhead pipe, Bryan manages to break the pipe loose and escape while killing the guards. Bryan meets the surprised Saint Clair standing at the elevator. Saint Clair pleads unsuccessfully for his life by saying "it was just business" and telling him that Kim will be at a pier. Bryan shoots him dead and sends the elevator up to the party. Bryan races to the the river, but misses the large yacht onto which Kim and other veiled women have been hustled as it cruises down the river. Bryan drives a stolen car against one-way traffic to gain position on the Pont Desarts bridge, then jumps onto the ship as it passes underneath. On board, an Arab sheikh lies on a bed in a robe as a group of women in veils are led in. Bryan fights his way through the sheikh's guards and enters the bedroom, where the sheikh has a curved knife held to Kim's throat. Bryan takes aim and it's a stand-off. The sheikh begins to say, "We can nego-" and Bryan shoots him in the head.
Back in the US, Kim is reunited with her mother. Lenore even hugs Bryan in thanks. Later, Bryan mysteriously collects Kim and takes her to a very nice home. After he knocks, he takes a step back. Pop star Sheerah opens the door and, over her shoulder, someone sits at a piano. Sheerah takes Kim by the arm while saying she had heard Kim wants to be a singer, and tells her, "let's see what you've got."