The novel and movie have significant differences. The novel takes place in the U.S., while the movie takes place in Paris. The following is the movie version.
The movie opens with a nighttime dinner scene at a country home with many of the main characters in the movie. It is revealed that the main character, Alex Beck, has been in medical school for nine years. What appears to be the following day Alex and his wife Margot drive to a secluded lake, Lake Charmaine, where they have childhood memories and a carved heart with their initials, M+A, on a tree near the lake. There, Margot uses Alex's pocketknife to add an additional hash mark to the many already present below the heart.
Thereafter they disrobe and romantically swim naked in the lake. After nightfall they end up snuggling together on a wooden raft in the center of the lake. After having a little argument about the cold relationship he has with his sister Anne, Margot comments she will "butt out" and swims back to the shore to check on the dog that was left in the car. Alex then hears Margot's cry for help, swims to shore, and is knocked unconscious as he attempts to exit the water using the ladder on the dock. He is last seen falling back into the water.
The next scene is eight years later when we see Alex arriving at the hospital where he works as a practicing pediatrician. While attending to a young girl and her parents Alex hears a commotion in the admission section and rushes to the scene. There he finds a thug-looking man named Bruno holding his son in his arms and demanding to see Alex. Alex is the only person there who Bruno will trust with his hemophiliac son who had fallen off the couch. Alex provides Bruno assurance and carries Bruno's son into the emergency room.
Alex meets his friend Helene, who is his sister's lover and domestic partner, for lunch in her restaurant, where he reads in a newspaper that two male bodies were uncovered near the Lake Charmaine. In his lunch conversation with Helene they discuss his plans to meet with Margot's mother in what has been a yearly meeting with her for the past eight years on the anniversary of Margot's murder. Alex also promises to stop by Helene's apartment after his visit with Margot's mother.
Alex stops and talks with Bruno on his way back to the hospital. Bruno provides Alex with his phone number on a card and offers Alex cash. Alex declines the cash to which Bruno suggests a DVD player or plasma TV. Alex refuses the gifts and tells Bruno he will update him when his son comes out of surgery.
When Alex returns to his office, he notices a new email message with the subject M+A followed by a row of about thirty slash marks (M+A///////////////////////////). He counts the slash marks and then opens the email. The email contains a link and the message with the words "open link," "anniversary," and "6:15 p.m." Alex attempts to immediately open the link, but it doesnt open. While Alex is still contemplating the email, he receives a message from his secretary that a police sergeant would like to speak to him.
In the next scene Alex has arrived at his sister's home where she is practicing jumping with her horse. They then meet with the police sergeant to discuss the case of the two men that were found buried near the lake. The discussion reveals several interesting points. The two men were found near where Margot was killed, but not on the family's property. A baseball bat was found buried with the two victims containing type B positive blood, the same blood type as Alex. And, even though Alex was cleared of his wife's murder and even though the murder was later attributed to a serial killer by the name of Serton, the police still proclaimed being puzzled by Alex's claim that he had been knocked unconscious into the water, yet was later found on the dock. Alex then submits to a blood DNA test. A nurse also shows up to take his blood.
That night he has several drinks while out walking his dog Nina. At home he stares at the email message on his computer screen and recalls the days with his wife including his childhood memories, his wedding, and her funeral where she was cremated.
The next day he is anxious all day for the email he is expecting at 6:15 p.m. on the apparent day of the anniversary of Margot's death. When he opens the link at 6:15, he observes that it is a web camera located at a shopping mall focused on a pair of escalators. While he is watching, he sees a woman who looks like Margot walk into the frame, turn to the camera, look at him, and slowly speak to the camera though Alex can only see her speak. She then turns around and walks off. Alex also receives a second email that contains the messages: "Same time + 2 hrs," "Username: Concert, Password: Olympia," and "Tell no one. They are watching."
Alex thereafter visits Margot's mother and father where he confronts Margot's father regarding how positive he was when he identified his daughter's body after the murder. Out of this conversation we learn that Margot's father was a captain in the police force at the time and that Alex never viewed or identified his wife. Margot's father is irritated by all of Alex's probing questions and eventually asks him to leave.
As promised at lunch the previous day Alex stops at Helene's apartment. While there, he tells Helene of the emails he has received and shows her the link to the web camera. In the discussion Alex mentions that the web camera must be located outside of France because it was dark in Paris while the web camera still depicted daylight.
While Alex is visiting Helene, Alex's sister Anna is participating in an equestrian meet sponsored by Gilbert Neuville in honor of his son who was an equestrian jumping champion who died eight years ago in 1997. When Anne shows up at Helene's apartment, she announces that she placed second, is very tired, and is going to bed. Helene whispers to Alex that she and Anne had had a little fight, that Anne was suspicious of Helene having an affair with a woman at work.
When Alex returns home, he again stares at his computer screen displaying the web camera at the shopping mall. Meanwhile two men at some unknown location are viewing and discussing the same two emails that Alex has received. One of them is knowledgeable of computers and states that the emails are anonymous and that the accounts are created at the time the emails are sent. He further states that he is tied into both of Alex's computers so that he would know when Alex goes on line. The other man, later identified as Bernard, states that he thinks he knows enough already.
The next morning several policemen are awaiting Alex when he arrives at the hospital. One of them is Captain Levkowitch who invites Alex to the police station where it would be more convenient to talk. At the police station Alex is asked about a woman named Juliette Langlois and is shown three photos of Margot bruised and beaten. Alex acknowledges that Juliette is his wife's middle name and that Langlois is the street where she lived growing up. Alex denies ever beating his wife and denies ever seeing the photos. Alex is next seen in a car with a high-powered woman attorney name Elysabeth Feldman, who was hired by Helene. Although Alex has never seen the photos that the police had in their possession, Alex told Elysabeth that while he was doing his residency in Bordeaux, Margot had been in an automobile accident with her girlfriend, Charlotte.
That afternoon Alex goes to visit Margot's old girlfriend Charlotte, a photographer, who he hasn't communicated with since his wife's death. Charlotte admonishes Alex for not staying in touch but Alex tells her that he just wasn't up to it. Alex then asks Charlotte about the automobile accident and the photos. Charlotte explains that there was no accident, that one day Margot showed up bruised and asked her to claim that she was in an accident with her, should Alex ever ask. She assured Alex that Margot had never lied to him regarding anything else.
When Alex returns to his apartment, he searches through boxes of Margot's stuff and finds her daily planner-calendar for 1997. In there he finds an appointment noted by the initials PF. When he calls the phone number, a woman answers the phone for a lawyer by the name of Pierre Ferrault, who is not in the office.
In the meantime Captain Levkowitch and his partner meet Margot's parents at their residence. Levkowitch shows the same three photos to Margot's mother that he had shown to Alex and inquires about Alex. Eventually Margot's father asks his wife to leave the room while he discusses further details. In that discussion Levkowitch and his partner make a case for Alex having murdered Margot for the 200,000-pound life insurance policy. In the conversation they disclose that the photos were from a safe deposit box under the name of Juliette Langlois and that the key was in the pocket of one of the men found at the lake.
Alex later meets up with Helene where they discuss the possibility of the police attempting to shake him up with the apparent emails from Margot. That evening the two of them attempt to open the web mail account with the username and password provided in the previous email. They are both unsuccessful, but discover that Alex's computer was actively being linked to another computer.
In the mean time, Bernard and a woman thug await Charlotte in her studio. When she returns from being out, they terrorize her and torture her regarding the whereabouts of Margot. After they receive a call that Helene has left Alex's apartment. Bernard shoots Charlotte and the two leave.
In the mean time Alex has taken his dog Nina for a walk. While thinking about the emails he has received, he gets an idea and runs with Nina to an Internet boutique several blocks away. The clerk will not allow Alex to enter with Nina, so he ties Nina to a bike rack and enters by himself. Another man, who arrived about the same time, allows Alex to go first and a minute or so later takes a seat at the computer to his right. Alex first enters the username "Concert" and password "Olympia" but then changes it to "U2" and "1995." The email account is valid and Alex finally gets to read the message he has been after. The message is to meet at a park near the bandstand the following afternoon at 5 p.m.
The next morning on the way to work Alex stops by the morgue to talk with the coroner regarding Margot's file and photos of her autopsy. He is told by the coroner that the records are not filed there and that he would have to formally request them, which he does.
In the mean time, the police have arrived at Charlotte's studio presumably called there by her associate. Her associate tells the police that when he had left work the day before, Charlotte was with a gentleman by the name of Alex Beck. The police immediately send a team of officers to Alex's apartment where they find some gloves in a trash can in the alley and a handgun taped to the backside of a desk.
Alex is next seen at work working with a patient when he receives a call from Helene's attorney, Elysabeth Feldman. She informs Alex of Charlotte's murder, that the police are on their way to the hospital, that she will meet him there as well, and to sit tight and not say anything. Alex checks out the hallways and exits where he sees police on their way. He then leaves the building through a window in his office, jumps to the roof of a van below, and then runs down the street pursued by the police. After a harrowing chase scene, Alex hides in a dumpster and calls Bruno. After another chase Bruno and his associates create a distraction and get Alex into his black Chevy Tahoe.
After losing Alex in the chase, Captain Levkowitch stops by his mother's place to drop off some groceries. There he speaks with his partner and admits to being the one who called Alex's attorney, Elysabeth Feldman. In his conversation Levkowitch goes through a list of reasons why he doesn't think Alex would have murdered Charlotte including hiding the murder weapon in his apartment behind a desk, his stopping at the coroner's office on the way to work, and even going to work at all. While his partner cautions him about the consequence of doing something that would re-open the murder case of the serial killer Serton, Levkowitch counters that he just wants to get to the truth.
Next, Alex and Bruno together go to the office of Pierre Ferrault where Alex questions Ferrault regarding his connection to Margot. Ferrault informs Alex that he had represented a man name Helio Gonzales who had been accused of killing Philippe Neuville, the son of the wealthy and well known Gilbert Neuville. Philippe and Helio had been seen together the night of Philippe's murder and Margot came forward as his alibi for the time of the murder. Margot had originally claimed that she had been professionally counseling Helio, but because no one was expected to believe that she would have been working at 11:00 p.m., Margot eventually admitted that she had had a two-month affair with Helio.
Alex, Bruno, and Bruno's associate next track down Helio. Although Helio is a jerk and enjoys humiliating Alex about having sex with his wife, Alex bluffs Helio telling him that he was with his wife the night Philippe was murdered and could thereby invalidate Helio's alibi and land Helio in jail. Helio falls for the bluff and confirms that he did not have an affair, simply went along with what Margot told him to say, and that she got him released from murder charges.
At 5 p.m. Alex shows up in at the park and waits on a bench near the bandstand as instructed in the email. However, unknown to Alex there are four men and a woman (including Bernard, the woman, and the man who sat next to Alex at the Internet boutique the previous evening) staking out the park from within and from a van parked near the entrance. The group initially mistake a woman with stroller and child for Margot. As Margot approaches Alex from behind, she sees a man talking on a walkie-talkie and hesitates to approach Alex any further. She then decides to abort the meeting and leaves through one of the gates. Because the gang of thugs had been concentrating on the woman with the stroller, they miss Margot leaving through the gate. Alex catches a glimpse of Margot leaving and attempts to catch up with her, but as he exits the gate, Bernard gives the go-ahead to take him captive. The four remaining thugs kidnap him in the van, hold him down, and begin to torture him for information. When the van stops at the next cross street, Bruno and his associate hijack the van and kill two of Alex's kidnappers, rescue Alex, and flee the scene in Bruno's black Chevy Tahoe.
In the meantime, Captain Levkowitch is parked at the morgue where he has obtained a copy of the autopsy report. While he is sitting in his car reading it and taking notes, he receives a call on his radio that Alex has purchased a ticket for a flight to Buenos Aires at 10:30 p.m.
Around this same time Bernard, the leader of the other four thugs, meets with his boss, Gilbert Neuville at Gilbert's home. At their meeting, Gilbert describes his attachment to his son, who he says, "rots underground, while she may still be alive." He then states that he "cannot accept that."
At the airport Margot is awaiting her flight. When she hears her boarding call, she stands and catches a glimpse of the televised news. The news flash indicates the murder of Charlotte Bertaud and the escape of suspect Alex Beck. Margot then proceeds to boarding where she changes her mind and leaves. In the meantime Captain Levkowitch and his partner learn from a clerk at flight check-in that the seat adjacent to Alex Beck was reserved by a woman named Caroline Perreire and that the tickets were purchased only five minutes apart. When they run to the appropriate gate which was starting to board passengers, they discover that neither Alex nor Caroline Perreire have boarded.
Alex, who is now at Brunos apartment, has no success in being contacted by Margot on the computer at Brunos apartment. He does, however call his attorney, Elysabeth Feldman, and tell her that he can prove he didn't kill Charlotte. In the meantime, Captain Levkowitch meets Helene walking home from work with Alex's dog. He tells her that he is probably the only one who thinks Alex is innocent and asks for her help in locating him. In their conversation he tells her that Margot's photos had been removed from the autopsy report and that according to the report, Margot was a heroin addict. Helen then begins to examine the report.
When Helene arrives home, she is greeted by a kiss from Anne who then follows her into the bathroom where Helene showers. While Helene showers, Anne tells Helene that she is the one who took the photos of Margot and that it was Philippe Neuville who had beaten her. Helene steps from the shower and a brief argument ensues. The phone then rings. The call is from Elysabeth Feldman who wants a photo of Alex's dog to establish Alex's alibi. When Elysabeth Feldman receives the photo of Nina, she provides it to the DA with statements establishing that Alex was at the Internet boutique at the time of Charlotte's murder and calls for the DA to do a press release that Alex is only wanted for questioning.
The following day Alex shows up at Helene's where Anne apologizes for all of the problems she has caused him by not telling anyone about the photos. Also present are Captain Levkowitch and his partner who apologize as well and provide Alex a copy of the autopsy report. Alex reads the report and recognizes that the dead woman was much taller than Margot. He has a few more words with everyone and then leaves the apartment. Captain Levkowitch and his partner ride down the elevator with Alex and continue to ask more questions as they also share some of their information. While they are talking in front of the apartment, a bouquet of flowers like those that grow near the lake are delivered to the apartment for Alex Beck in care of Helene.
Next, with the flowers in the back seat of his car Alex drives to Margot's parents' house. The front door is slightly open, so Alex enters. When inside, he hears Margot's father tell him that he is in the kitchen. As Alex walks into the kitchen Margot's father pulls a gun on him from behind, frisks him, and then tells him to have a seat in the living room. He pours himself a stiff drink and proceeds to provide an account to Alex. As Margot's father tells the story the audience is provided scenes to accompany the story.
Two months before the event at Lake Charmaine, a boy from the children's trust came to Margot distressed and told her he had been molested by Philippe Neuville. This prompted Margot to invite Philippe to her home in the country where she confronted him. The plan was for Margot to extract a confession for her father who was waiting outside. However, when confronted, Philippe violently retaliated and was beating Margot mercilessly when Margot's father entered the house with a shotgun. He immediately became enraged and shot Philippe with both barrels.
As Margots father continues to tell the story, the scene changes to another location where the police are listening to the story via a wire worn by Alex. At a point in the story, the wire gets static and the police can no longer hear the story continue. When the signal becomes clear again, we learn that Margot's father then disposed of the body to implicate a kid named Helio Gonzales who lived at the home of the childrens' trust. Margot, however, would not accept Helio taking the wrap for something he didn't do, so she provided an alibi to get him off. Then, to protect herself Margot placed the photos taken by Anne in a safe deposit box and contacted Gilbert Neuville to warn him that she was holding incriminating evidence against Philippe.
Margot's father then tells Alex that Neuville had half of the police force and politicians on his payroll including him. He also knew how Neuville would respond to Margot and thereafter tapped Neuville's phones where he later learned that Neuville had hired two men to kill Margot and retrieve the evidence. To outwit Neuville, he then paid one of the two men to help him with a different plan. He then staged Margot's death by using the body of a drug user that had been stabbed. He then mutilated her face so that no one could know it wasn't Margot. He further staged the scene to appear like that of a wanted serial killer.
In brief, he dragged Alex from the lake, loaded Margot's body into his car, killed both men hired by Neuville, and buried them around 100 meters away. He also told Margot that Alex had died so that Margot would leave and not return. He then sent her by air to Spain and has not contacted her since. He claimed his only mistake was that he forgot to remove the safe deposit box key from the man who had taken it from Margot's purse.
When Alex asked Margot's father why he didnt seek help from the authorities, Margot's father claimed that he had already crossed the line. He then provided Alex an envelope containing the details of all the crooked matters he had had with Neuville. He then began to discuss Alex's father, who had also worked as a trainer at the stables.
Six months earlier Alex's father had also discovered that Philippe had been molesting kids and had demanded that Margot's father type up a complaint. However, when Gilbert Neuville read the complaint, he simply tore it up and then had Alex's father killed in a staged hunting accident. At this point, Margot's father puts down the gun. Enraged by the story, Alex picks up the gun, aims it at Margot's father, and begins to squeeze the trigger. Apparently regaining his senses, Alex leaves the house where he is greeted by a swat team and Captain Levkowitch. As Alex closes the door behind him, we hear a shot. Alex then hands over the envelope to the police, removes the wire from under his shirt, and walks off.
In the next scene, Gilbert Neuville is placed under arrest while attending the jumping show in his son's honor. As he is being escorted away, a rider falls on a fourth jump of a tight series.
We next see Alex on his way to Lake Charmaine. As he drives along he has a flashback of the story provided by Margot's father. We again see the scene where Margot's father searches Alex. We then see and hear what transpired when the voice on the wire was drowned out with static. We see that Margot's father had deliberately turned up the volume of static on the TV to suppress the conversation. He then very quietly tells Alex that he knows he is wearing a wire and will tell him what really happened. Instead of Margots father shooting Philippe as before, we again see Philippe punching and kicking Margot, but then leaving through a door. We then see Margot struggling to pull herself to her feet, grabbing several shells from a box in a drawer, inserting the shells into a shotgun, following Philippe out through the same door, and then when he turns around, she shoots him twice. Later we see Margot's father arrive at the scene.
At Lake Charmaine, Alex stands before the tree containing the heart, their initials, and hash marks. As he stands there, Margot walks up behind him. When he hears her footsteps, he drops to his knees and begins to cry. Margot walks up behind him and the camera pans upwards. The movie ends with the two of them as children, kissing, holding hands and sitting down on the pier together.