The movie begins with a lively house party in a French suburb, where our protagonist, Alex, and his wife, Margot, are having fun with their family. The following morning, the couple drives to a secluded lake named Lake Charmaine, which they used to often visit as kids. There, we see that they have carved "M+A" into a tree; there are several slash marks in the tree, and Margot adds another one using her pocket knife.
Afterward, the couple undresses and jumps into the lake to swim. They spend the entire day having fun, and in the evening, they lie down on the dock. Here, Margot asks Alex why he is distant from his sister, and urges him to mend their relationship.
However, he refuses to discuss it, leading to an argument between them. A frustrated Margot then gets up and swims to the shore. But moments later, Alex suddenly hears her scream, so he panics and rushes to find her.
Unfortunately, as soon as he reaches the shore, someone hits him on the head, causing him to fall into the water. The scene then shifts to 8 years later, where we see that Alex is now a pediatrician. It turns out that Margot has been dead since the lake incident; however, he is still grieving for her and unable to move on.
Currently, Alex is checking on a patient, when he suddenly hears noises outside and goes to investigate. It turns out that a local thug named Bruno has come to have his son treated, and he is refusing to see anyone except Alex. It is revealed that the child is hemophiliac, and Alex has been treating him for the past three years.
Seeing Bruno causing havoc, Alex calms him down and immediately takes his son for a checkup. Afterward, during his lunch break, Alex meets a woman named Helene, who is his close friend and also his sister's lover. He informs her that he plans to meet Margot’s mother soon because her death anniversary is approaching.
Hearing this, Helene feels annoyed and advises him to move on from Margot, as it has already been 8 years. Later, when Alex returns to his office, he suddenly receives an email with the subject "M+A," followed by several slash marks. The email contains a link, but when he tries to open it, he is unable to do so.
Just then, his assistant arrives and informs him that a police officer wants to meet him. In the next scene, Alex arrives at his sister, Anne’s place, where she is practicing horseback riding. Afterward, the two head inside and meet a police officer.
The officer informs them that while digging near Lake Charmaine, two male bodies have been found in the forest. The men remain unidentified; so they need the siblings' signatures to excavate further on the family's property. The officer adds that they found a baseball bat with the bodies, containing type B positive blood, a rare blood type Alex possesses, pissing him off.
In the past, Alex was cleared of Margot's murder, because the crime was later linked to a serial killer, who only kills woman. However, it doesn't make sense that Alex got knocked unconscious into the water, and later found unconscious on the docks, or who made the anonymous 911 call. Therefore, the police officer requests a DNA test on Alex to investigate further.
Alex becomes angry and can't believe he is being accused of killing his wife. The following day, it is the anniversary of Margot’s death, and Alex is feeling very restless and sad. He checks his email again and opens the previous link, which now successfully opens.
It turns out to be a webcam located at a shopping mall. While he is watching, he is horrified to see a woman who looks exactly like Margot in the footage. She looks at him with a worried expression, before suddenly walking away.
During this time, Alex receives another email in which she mentions a username and password for an email account, warning him not to tell anyone, as they are watching her every move. That afternoon, Alex goes to visit Margot’s parents. He talks with Margot’s father, who was the chief officer in charge of handling Margot’s case.
Alex asks him how he identified his daughter’s body, and if her face was clearly visible. Margot’s father claims that her face was bruised, but he could still recognize her. Alex continues to probe him with further questions, but this irritates and offends his father-in-law that he eventually asks Alex to leave.
In the evening, Alex goes straight to Helene's apartment, and shows her the email and the ongoing CCTV footage. Helene wonders where the place could be, but he claims that it is not in France because it's still daylight. During this time, we see Anne participating in an equestrian meet sponsored by a rich man named Gilbert Neuville.
The event is in honor of his son, Philippe Neuville, who was an equestrian and died eight years ago. Later, when Alex returns home, he continues to watch the webcam footage, feeling confused after 8 years. Meanwhile, in another location, a guy named Bernard and his accomplice are also watching the same footage.
It turns out that they have hacked Alex’s computer and are updated about each email he receives. The following day, while heading to work, Alex is stopped by a detective named Levkowitch who wants to interrogate him. Levkowitch asks him if he used to beat his late wife.
Hearing this, Alex is horrified and denies ever laying a hand on her. The detective then shows him some pictures in which Margot is badly bruised all over her body, but Alex maintains his claim that he never hurt her. Later, Alex meets with a high-profile attorney named Elysabeth, who has been hired by Helene.
He tells her about the photos the police showed him and claims that he never hurt his wife. Alex also explains that Margot was once in a serious car accident, and suggests that maybe she took those pictures for her insurance claim. After this, Alex goes to meet a photographer named Charlotte, who used to be Margot’s best friend.
He asks if she took those bruised pictures, but Charlotte denies it. He then inquires about the accident because Charlotte was with Margot that day. But she admits that there was no accident, and that Margot just showed up bruised, and Margot asked her to claim that she was in an accident with her.
Alex is surprised to hear all this. When Alex gets back to his apartment, he looks through Margot's old boxes and finds her daily planner from eight years ago. There, he sees an appointment marked with the initials PF.
When he calls the number, a woman answers for a lawyer named Pierre Ferrault, who is not in the office. In the meantime, Detective Levkowitch and his partner meet Margot's parents and show them the bruised pictures of their daughter. They suspect that Alex might be behind this, but Margot’s mother claims that he would never do such a thing.
Still, Detective Levkowitch claims that Alex might have murdered Margot for the £200,000 life insurance policy. They are also confused about how he got to the shore if he was knocked unconscious. The detective discloses that these photos were found in a safe deposit box under her name, and that the key was in the pocket of one of the dead men found at the lake.
Hearing this, Margot’s father is left speechless. That evening, Bernard, and a female assassin approach Charlotte in her studio. They attack her and demand information about Margot's whereabouts.
Meanwhile, Alex is at home with Helene, trying to open the webmail account using the username and password provided in the previous email. This doesn't work, but then they discover that Alex's computer is being hacked by someone else. On the other hand, Bernard and his accomplice eventually kill Charlotte after realizing that she doesn't know anything.
Later that night, Alex is walking his dog when he suddenly comes up with an idea. So, he goes to a cybercafé, but since the clerk doesn't allow the dog to enter, he ties him outside. Once inside, Alex tries a different account than the one on the email, and finally, it works.
In the email, Margot has asked him to meet at a park near the bandstand the following afternoon at 5 p. m. This makes him very happy, as he realizes that his wife must still be alive.
The next morning, Alex goes to talk with a coroner to discuss Margot’s autopsy report. At the time of her death, he was unconscious in the hospital, so he couldn't see his wife's body. The coroner informs him that the records files are not available here, so Alex makes a formal request to access these files.
In the meantime, the cops find the dead body of Charlotte, and from her employee, they discover that she was last seen with Alex before she died. With this information, the cops immediately storm Alex’s apartment, where they find gloves in the trashcan, and a handgun taped to the backside of a desk. Seeing this, they conclude that he is the killer and head to his office to arrest him.
On the other hand, Alex is in his office when the high-profile attorney, Elysabeth, calls to inform him that Charlotte is dead, and the cops are coming to arrest him for the murder. Hearing this, he panics and decides to flee, as he has to meet Margot today. The cops also notice him and begin to chase him.
Luckily, Alex eventually outsmarts them, and hides in a trash can. After the cops leave, he calls one of his patient’s father, Bruno, and asks for his help. Bruno immediately arrives to help him, as he feels indebted to Alex for saving his son's life.
Afterward, the two visit the office of Pierre Ferrault, the lawyer whose number was found in Margot’s day planner. Alex asks him about his connection with Margot. Here, Pierre admits that he had represented a local thug named Helio Gonzales, who was accused of killing Philip Neuville, the son of well-known Gilbert Neuville.
Margot came forward as Helio the suspect's alibi on the night of Philippe's murder, claiming she was counseling him. At first, she claimed that she was professionally counseling Helio, but later admitted they were having an affair for two months, and Alex cannot believe that his wife cheated on him. Later, he and Bruno track down Helio, asking about his relationship with Margot, and Helio continuously boasts about how he slept with Margot.
Alex then lies and claims it’s not true, because she was with him the night Philip Neuville was murdered. He states that he can prove it in court and threatens to send Helio to jail. Feeling threatened, Helio admits that he actually didn't have an affair with her and that he didn't kill Philip either.
Margot only helped him to be released from the murder charges. Later, at 5 pm, Alex arrives at the park to meet Margot. However, unknown to him, Bernard and his accomplices are also present to apprehend her.
Shortly after, we see Margot actually arrive wearing sunglasses, but she notices the assassins waiting for an opportunity to get to her. This scares her so she decides to leave without meeting Alex. During this, Alex catches a glimpse of her from behind and starts to follow her.
Unfortunately, he is soon captured by Bernard’s group. The thugs force him into a van, where they begin to torture him. The female assassin demands to know Margot's whereabouts from Alex, but he insists he has no clue.
She tries to kill him, but then… Bruno arrives with his accomplice just in time and murders two of the thugs. They then rescue Alex and escape in their van. Meanwhile, Detective Levkowitch reaches the morgue to collect Margot's autopsy reports.
But while going through the report, he suddenly receives information that Alex has booked a flight to Argentina, departing within a few hours. In another scene, Bernard meets with his boss, who turns out to be none other than Gilbert Neuville. Gilbert claims that he cannot accept the fact that his son is dead, while his killer, Margot, may still be alive.
So, he orders Bernard to find her as soon as possible and put an end to her. Elsewhere, the police hurry to the airport to apprehend Alex, but they find that the ticket booking was made by Margot, who reserved two seats. They hurry to catch her before she boards the plane.
Meanwhile, Margot is at the boarding gate when she sees the news on TV about her best friend Charlotte's death and the fact that her husband is the prime suspect. This devastates her, so she changes her mind, and decides to help her husband get out of this situation, leaving the airport before the cops can get to her. That evening, Alex calls Elysabeth and informs her that he can prove he didn't kill Charlotte.
Meanwhile, Detective Levkowitch meets with Helene and asserts that he knows Alex didn't murder his wife. He shows her the autopsy report, and reveals that Margot's photos have been removed from all the files. Moreover, the report indicates that she was a heroin addict, despite the fact that Margot had never used drugs in her life.
When Helene returns home, Anne finally admits to her that she is the one who took the bruised photos of Margot, and reveals that it was Philip Neuville who had beaten her. Helene is furious, and demands to know why Anne didn't reveal this earlier. Anne tearfully apologizes and explains that Margot asked her not to.
Here Helene accuses her of protecting Gilbert Neuville, as she works for him and rides his horses, and Anne ends up slapping Helene. Meanwhile, Elysabeth meets with a district attorney, and shows him a picture of Alex’s dog outside the cybercafe. She explains that there are witnesses to prove that Alex was in the cybercafe the night of Charlotte's murder, which proves that Alex is innocent.
The next day, Anne confesses everything to her brother, and apologizes for not revealing the truth about the photos earlier. Detective Levkowitch also arrives there, and shows him the autopsy report. As Alex reviews it, he is shocked to discover that the deceased woman is significantly taller than Margot.
However, the dead woman's photos are missing, believed to have been stolen by Margot's father, who was the chief cop at the time. In the next scene, Alex decides to confront his father-in-law. But then, upon arriving at the house, Margot’s father holds him at gunpoint, and instructs him to sit down.
He then reveals that two months before the lake incident, a child from the Neuville Foundation orphanage approached Margot, and informed her that Philip Neuville had been assaulting the children there. Therefore, Margot invited Philippe to her house to confront him about his actions. Her father was also outside the door, waiting for Philippe to confess so that he could arrest him.
But instead, Philippe became aggressive and brutally started beating her. The father couldn't stand seeing her daughter get beaten up like that, so he killed Philip with a shotgun. During this, it is revealed that the police are listening to Margot’s father’s confession via a wire worn by Alex.
But then, their wiretap suddenly cuts off and is filled with noise. Moments later, the microphone starts working again. The father then disposed of the body to frame the thug, Helio Gonzales, who lived at the home of the children's trust.
However, Margot refused to let Helio take the blame for something he didn't do, so she provided an alibi to get him off. She then placed the photos of herself beaten up in a safe deposit box, and contacted Gilbert to warn him that she was holding incriminating evidence against his son Philippe. Margot’s father informs Alex that Gilbert accused Margot of killing Philippe, and hired two men to kill Margot that night at the lake.
However, Margot’s father knew what Gilbert was going to do, so he arrived and killed the two thugs, saving his daughter. Margot's father then buries their bodies nearby. He also rescued an unconscious Alex from the water, and left him on the shore.
After that, he searched for a dead body, and eventually found a dead junkie woman who had been stabbed. He dumbed the body 100km away, mutilated her face, and placed Margot’s ring on her finger to make it look like her. He realized that Gilbert won’t let Margot go if he knew she was still alive.
He also lied to Margot that Alex had died, so she would not return. He ended her to Spain, and has not contacted her since. Margot’s father hands Alex an envelope containing detailed information about all the crimes Gilbert has committed.
He then discusses Alex's father, who used to work as a trainer at Gilbert’s stables. It turns out that Alex's father had uncovered Philippe's abuse of children, and had filed a complaint against him that Margot's father wrote. Due to this, Gilbert, who is so powerful, simply tore the complaint into pieces, and killed Alex's father in a staged hunting accident.
Hearing this, Alex becomes so enraged that he points a gun at his father-in-law. Meanwhile, the police arrive to arrest the culprit just as Alex leaves the house with Gilbert’s crime envelope, and we hear that Margot’s father pulls the trigger on himself. In the aftermath, the police finally arrest Gilbert for the crime he has committed over the years.
Meanwhile, Alex is driving to Lake Charmaine while recalling what his father-in-law said. It is at this point that we learn Margot’s father actually knew about the wire recorder, so he turned up the static on the TV to actually tell the truth. It turns out that instead of her father, Margot was the one who grabbed the shotgun and killed Philippe.
To protect his daughter, Margot's father took the blame upon himself, and even sacrificed his life. Upon arriving at the lake, Alex can’t hold back his tears; he stands before the tree where they had carved their initials. Suddenly, Margot emerges from the bushes, and upon hearing her footsteps, he drops to his knees.
After 8 long years apart, they are finally reunited at their special spot.
