On March 9, 1928, we see Christine Collins (Angelina Jolie) wake up in the morning and wake up her son Walter (Gattlin Griffith), and after measuring his height against the wall, gets him ready for the school day. She drops him off at school and goes to her job as a supervising phone operator at a Los Angeles telephone company. At the end of the day, she picks Walter up at school and he tells her that he got into a fight when another student said that Walter's father hated him. Christine corrects him by saying his father did not even know him, so how could he hate him. Christine then explains that Walter's father was merely frightened of the responsibility that came with Walter and left before he would have to deal with it and Walter smiled and accepted this explanation. The next day Christine gets a phone call saying that she has to come in since one of the other girls was sick. Eventually she agrees disappointing Walter with the news that they have to go to the movies tomorrow to which he sighs but nods. She tells him that there's a sandwich for him in the fridge and reminds him that a couple neighborhood girls will be by later to check on him, and she then heads out.
At the phone company, Christine is getting ready to leave when her boss Ben Harris (Frank Wood) stops her. He notes that she has done a great job as a supervisor and says that there may be an opening for a full-time assistant manager. She thanks him for the opportunity and they agree to discuss it over dinner the next night (although he obviously wants it to be more date and less business). Eager to get home she excuses herself abruptly so she does not miss the trolley and returns to find Walter missing. She searches the entire neighborhood and asks the neighborhood girls if they have seen him, but to no avail. She calls the police, but is told by the man who answers, Detective Lester Ybarra (Michael Kelly) that most missing children turn up by morning and she should not worry, he's probably just somewhere in the neighborhood and lost track of time, and that she is saying nothing different from what other mother's who have called has said. The next morning a police car comes and two officers take down her information and Walters description and take a recent photo of him.
During the next few weeks, we see the investigation in various stages, led by captain J.J. Jones (Jeffrey Donovan), the head of the missing persons department. While they are conducting their investigation, Christine is doing her own investigation, calling various cities as far away as Las Vegas on a regular basis, and seeing if any children fitting Walter's description have been found. At the same time, we see radio preacher Rev. Gustav Briegleb (John Malkovich) praying for Christine and for the safe return of Walter with his congregation while at the same time lambasting the corrupt and lazy police force. After five months of this, though, they are no closer to finding Walter than when they started. Meanwhile, in a town in Illinois called Dekalb, we see a man and his son (Devon Conti) at a hole-in-the-wall diner. The man pretends to reach for his wallet to pay but realizes he forgot it at home. The owner is angry but decides to give him ten minutes to get to his house, pick up his wallet and return to pay. The man agrees to leave his son as collateral which the boy starts to protest, and it is apparent to the audience that this is an obvious con. After two hours of waiting the owner decides to call the cops.
Back in San Francisco, we see Christine moving listlessly through her job when Jones and Lester show up with some good news for her -- they have found Walter. After many congratulations at the company, Jones and Lester along with Chief James E. Davis (Colm Feore) escort Christine to the train station where in the midst of a big media circus she is reunited with.... the boy from the diner. She states that he is not her son, and Jones explains how five months in captivity can both change how a person looks. Jones then asks the boy who he is and his address, and the boy gives the correct answers, but in a tone of voice that makes them sound like lines memorized for a play. After debating the issue, Christine eventually states that maybe Jones is right and agrees to give the boy a home and poses for an awkward photo with "Walter" for the newspapers.
That night Christine and the boy are having dinner and she begins noticing things. First, the boy does not use manners, something Walter had been studious about. Second, she notices as she is toweling him off after a bath that he has been circumcised. She drags the boy to where she had measured Walter back in March, only to find that this child is three inches shorter than Walter was. This prompts her to visit Jones, who assures her again that they found the right boy -- almost demanding it, actually and after she leaves, sends a doctor to her house to verify. Dr. Earl W. Tarr (Peter Gerety) arrives for the examination, and after listening to Christines story, he gives her the same nonsense Jones gave her, only now including that the spine can shrink in stressful times and that the kidnappers may have circumcised the boy themselves. She still refuses to believe this, though. That night, when the boy is going to bed, he smugly says "Good night mommy", causing Christine to fly into a rage, flinging a plate against a wall and screaming that he is NOT her son and that she wants HER son back. Later, after the boy is asleep, she goes into the room and apologizes to him for yelling but again asserts that they are not related.
The next morning, Christine is woken by a phone call from Gustav, who tells her to read the front page of the times and then stop by his church for breakfast. Sure enough, the days article is about her denying that the boy is her son despite the best efforts of the valiant officers and indisputable evidence that the boy is her son and how the police fear for the child's safety etc etc. Gustav then tells Christine the story of how Davis became chief and his first action was to give the biggest, toughest cops machine guns and then let them kill all the criminals they could -- not to eliminate crime, but to eliminate the competition because the police department wanted control of the smuggling and gambling. He then warns her of what she may be getting herself into, but notes that if she pulls this off, she could be the last woman to have the wrong child returned.
We then see Christine at Walter's dentist, who notes that Walter had a piece of muscle preventing the gap between his teeth from closing, which this boy does not have -- his teeth are even, something which he is eager to put in writing for her. The case is the same with school, where Walter cant remember his despite her teaching Walter for over a year. And when "Walter" is unable to find his assigned desk this gives the teacher ample cause of her own to put her belief that this is not Walter Collins in writing, along with a promise to testify in court. Soon, we see Christine on the steps of a courthouse telling reporters that the police were wrong and she has proof of it. Naturally, this angers Chief Davis to no end and he orders Jones to take care of it. Jones has Christine and the boy brought in and after a heated "conference" wherein he calls her either delusional and a liar or that perhaps she became adapted to being free from having responsibility and is now lazy and wants to be free again. He asks her one more time if that boy is her son. When she says no, he opens his door and nods to two people and they take her away to an insane asylum. Here she is subjected to a power hose shower and a very crude female examination and is told she is being held for her own safety and the safety of the boy until such time she can demonstrate that she is sane and capable of taking care of herself and the child.
Meanwhile, off in the desert somewhere, we see Detective Lester Ybarra investigating a missing child report regarding a kid from Canada. He pulls over at one point to ask a stranded motorist for directions to the Northcott Ranch. The motorist pales a little and walks towards the back of his truck where a shotgun is in the bed but decides not to use it and instead eagerly gives him the directions. Lester thanks him and heads on his way to the seemingly deserted homestead. After poking around some he finds the boy, and while chasing him through the house, happens to see in a photo on the wall that the motorist was Gordon Northcott (Jason Butler Harner), the owner. Catching the boy Sanford Clark (Eddie Alderson), he informs him that he's going back home to Canada and at the same time we see Gordon in a train station, trying to buy a one way train ticket to Canada.
Back with Christine (who has missed her appointment to appear on Gustav's show), we see her having her first breakfast in the asylum. She meets a woman named Carol (Amy Ryan) who is both sane and aware of her situation. Like Christine, Carol and a number of other women there, were brought in under a "Code 12" designation. Which is loosely defined as essentially someone who has peed off a police officer in some way. Carol's reason, since she was someone who "worked" nights, when a customer got a little rough and she filed a complaint, she found out that the customer was a cop. The available options according to Carol is behave by the asylum's rules and stay in one piece by keeping your mouth shut; or fight the system and end up with repetitive shock treatments. After breakfast, Christine has her first appointment with the head doctor, Dr. Johnathan Steele (Denis O'Hare). Christine tries to behave calmly and rationally, but his dismissive treatment of her and his accusations of her lying, coupled with the photographic evidence of her posing with the boy in a front-page article about mother and son reunited, flusters her. She eventually starts stammering and tries to explain which only ends makes him accuse her of not being able to distinguish reality from lies and makes her look more insane, much to her dismay.
Meanwhile, we see Sanford in a waiting room somewhere with other kids, one of whom keeps banging a ruler up and down on his knee, causing Sanford to get flashbacks of a man with an axe hitting something and covering himself with blood every time. After he is told he is being deported back to Canada, he demands to see Lester. Lester angry at being stuck at work longer and wanting to go home, is abrupt and callous with Sanford until he hears the story of how the boy had intended to only stay with his cousin Gordon at his ranch for a little while, but was eventually forced to stay there. Lester lights a cigarette and starts asking what his point is and if that was some excuse for being here illegally, but Sanford tells him he couldnt leave because he was afraid for his life, and tells him all about what he and Gordon did. According to Sanford, every now and then they would go driving in the city and pick up a boy who was alone by saying that his parents were in the hospital and they needed him to come with them now. In addition, if just an adult asked the kid would automatically refuse to get in, since they wouldn't take a ride from strangers. However, if they saw that there was already a kid in the car -- a kid like Sanford, for instance -- then it must be okay to get in. After they had the boy, they would take him back to the ranch and lock him up inside the chicken coop on the grounds, sometimes with as many as four or five others. Once things started to get crowded, Gordon would take some of the boys and hack them to death with an axe.... except sometimes he wouldn't go all the way and he would force Sanford to finish. In either case, Sanford would then have to dig a grave and bury the bodies on the grounds. By the end of this story, Sanford is sobbing and Lester is in shock, with his cigarette having burnt all the way down without having been smoked. Pulling himself together, Lester asks if Sanford would remember the faces of those boys and Sanford nods yes. Lester pulls out a stack of pictures of missing boys and Sanford starts going through them. He puts the ones he recognizes face up on the table in front of him, eventually creating a pile of ten or fifteen pictures, with only a few rejects. Lester starts going through the recognized pile, looking at the names on the back. He pulls out one and asks Sanford if he is positive, and Sanford nods. Lester whispers, "Jesus Christ," and puts the picture -- labeled Walter Collins Age 9 -- on the table. Lester heads out of the room, and after grabbing Sanford, he grabs two other cops and tells them he'll explain on the way, and heads out the building.
The next day, Christine is told by a nurse and orderly that she has to take her medicine. Never told what this medication is, she refuses to take it until she talks to her doctor. After being denied permission to speak to him, she races down the hall, bursts into his office, and demands to know what is going on. He asks her why she thinks she does not need it. She says because she is not ill. He tells her than if she is willing to sign a contract saying that she was wrong and the police found the right boy, then she would be considered not ill and could go home. She refuses and grows angry, causing the doctor to yell for an orderly, who drags Christine out of the room kicking and screaming and forces the medication into her mouth. Carol, who is near all this, comes to Christines defense by fighting off several of the nurses and orderlies and even going so far as to punch the doctor when he tells her to back off whore. This unruly conduct gets her some time in the shock treatment room. When Christine checks on her later, Carol tells Christine that the important thing is to keep fighting, and as for the doctors and orderlies...."Fuck them and the horse they rode in on." Christine laughs at this unladylike remark and Carol says that there is a place and time for being ladylike, and this is neither.
Meanwhile Lester, the 2 officers and Sanford, reach the ranch. Lester grabs a shovel and tells Sanford to show them where the bodies are buried. He takes them to a spot that may once have had a small coop but now holds just the base of it, the rest having perished in a fire. Lester tells Sanford to start digging, which he reluctantly does, eventually unearthing first a shoe with a skeletal foot, then a hand, and last a pair of ribcages. As Lester's partners recoil in shock and disgust, Lester steps forward and tells Sanford to stop, which after a brief struggle he does, collapsing in the dirt and crying. Later Jones is talking with Lester on the phone about Lester's discovery and the implications, especially those in the Collins case, when Gustav shows up with an army of lawyers demanding Christine. Jones quickly tells Lester to just get Sanford to the station and goes out to confront Gustav. Fortunately, Gustav is able to get Christine's current location. The day after, we see Christine at roll call (where the patients stand outside their doors to be counted). Steele asks Christine if she's ready to sign the contract and she says proudly, "Fuck you and the horse you rode in on." Steele orders immediate shock treatment and heads for his office just as Gustav is barging in with his army. Gustav demands the head nurse to get the doctor in charge of Christine, and after showing the doctor the latest article in the news, an order is given to nurses in the shock room to let Christine go and bring her to Steele's office. He tells her that she can leave, which she does without knowing quite why. As she departs, as is walking down the street we hear the newsie on the street corner shouting Extra extra read all about it Bodies of missing children found at ranch and Collins child believed dead". Christine hears the headline and faints, but Gustav is suddenly there to catch her and take her home.
Sometime later, we see the fake Walter interviewed by Lester. He claims innocence, but Lester says that given the Gibson boy's death, he could be charged with accessory to murder. The kid does not believe it, but when Lester mentions that Sanford is going to jail at only fifteen he starts talking. Meanwhile, in Vancouver we see Gordon arriving home at his sisters house. His sister opens the door and stunned at seeing her brother stiffly hugs him. Gordon announces he is going to use her shower and after he goes upstairs, she tells her husband to call the Mounties.
Back in the states, Christine receives a visit from Gustav. He has brought a friend with him who he introduces as S.S. Hahn (Geoffrey Pierson). Gustav asks Caroline what her next step is and she brings up the idea of a civil case against the whole department. In turn, Gustav notes that he tracked down the only lawyer to go up against the city four times in civil cases and wins, but that he was much costlier than Gustav could afford. Christine nods sadly, at which point Hahn says that was why he agreed to do this case pro-bono. On top of that news and the subpoenas being filed against the department, including Jones and the chief, Christine and her new friends also manage to get all the women in the asylum that were placed there under Code 12 released -- including Carol, who shares a warm smile with Christine as she makes her way out. About two weeks after this all started, on October 4, 1928, we see Christine and Gustav heading into the courthouse amidst a swarm of people all carrying signs insulting the integrity of the LAPD. Inside the court, we see representatives of all parties gathered except for the LAPD. A recess is called, to the committee heads displeasure, and Christine is taken to a separate courtroom where the trial of Gordon is being held and where he enters a plea of "Not guilty". In the audience, a woman taps Christine on the shoulder and identifies herself as Leanne Clay (Lily Knight), a mother of one of the boys Gordon killed, which gives the two women an instant connection. On the same day, we see the fake Walter -- really a boy named Arthur Hutchins -- being reunited with his real mother by a smiling Chief Davis, who wants the press to note that the LAPD does many good deeds despite the bad deeds undertaken by a minority of the department. Unfortunately for him, this line gets derailed once the boy pipes up that it was the police who told him to be Walter Collins, which has Davis all but shoving the boy onto the train just to get rid of him.
At this point, clips from each trial are shown. People such as the Chief Davis, Walter's teacher, and Walter's dentist testifying. Most prominently shown is a clip of Captain Jones being interviewed wherein Hahn gets him to admit that he sent Christine to the mental hospital without warrant or cause. Eventually, the trials are concluded and verdicts are reached. For Gordon's case, it ends with Gordon being found guilty, at which point he pauses to say that he hates everyone in the courtroom except for Christine, who he calls a real classy lady. He is sentenced to solitary confinement at San Quentin prison until October 2nd, 1930, at which point he will be hung from the neck until dead. In the civil trial, the verdict is that Jones shall be permanently suspended, and Davis shall be demoted from his position as chief. Also an internal review of the steps needed to allow someone to be committed shall be undertaken so that events such as those which allowed Christine and the other women to be committed so easily shall not happen again.
Two years later, on September 30th, 1930, we see Christine at work. She takes her ten-minute "break", which consists of calls to missing child departments, when she is interrupted by none other than Hahn, her old lawyer friend. He informs her that Gordon has requested that he see her so he can tell her face to face what really happened to Walter. She goes down the next day but once the two are together, Gordon clams up and tells her to go away and that he doesnt want to see her. He says that he has confessed to God and been forgiven by Him and that he does not want to talk now, lest he go to Hell. Christine demands to know what happened and that he tell the truth, even going so far as to throw him against the window and slapping him to get him to talk, but it has no effect. She stays for the hanging the next day. Gordon is calm and detached, making conversation with the reverend and saying he has no final statement. He loses control on the steps to the noose, though, and has to be carried up by two guards. A bag is placed over his head and the noose is fitted as straps are tightened around his body to ensure a minimum of struggling, lest the rope break and the whole thing be ruined. As the time approaches, Gordon strangely begins to sing "Silent Night" and the warden nods and the floor drops. He jerks briefly, but after a minute is still.
Years pass. We next see Christine on February 27th, 1935. Her operator friends are holding an Oscar party at one of their houses, but Christine declines, saying that she has too much work. Ben pops his head in her office and also asks, but she declines... although after a moment, she tells him that if "It Happened One Night" (her pick for best picture) wins best picture over his pick, "Cleopatra", then she'll have dinner with him tomorrow night. Sure enough, that is just what happens. As she grins in her office, the phone rings. She picks up expecting it to be Ben, but is instead surprised by Leanne Clay, who says that they found one of the missing boys. Christine runs to the Lincoln Heights precinct and meets with Leanne and her husband, who are watching in a viewing room as Leanne's son David (Asher Axe) is interviewed by Lester. David says that he was there with four other boys -- two brothers named Winslow, a kid named Joseph... and Walter. When asked how he remembered Walters full name so vividly, he answered, because of what happened to him. The boys staged a breakout thanks to some loose fence on the coop. The Winslow boys, Joseph, and Walter, all got out quick, but David was stuck on the fence. As Walter ran back to get him loose, the lights are turned on in the house and Gordon and Sanford realize that the kids are missing. Walter frees David and the two run for it as Gordon gave chase and fired his shotgun. They all went in separate directions, and David never saw any of the other boys again. Eventually he found a house where some old lady took him in, and that is where he had been staying until coming forward. Lester asks why he waited so long, and David says that he was scared that he'd be blamed for some of the deaths or that he'd be in some kind of trouble, and that he missed his mom and dad (at which point his parents come in and engulf him in a huge family hug). Lester and Christine head outside and he consoles Christine, saying that what Walter did was incredibly brave. She agrees and he asks what she will do now. She smiles at him and says that if David escaped and survived, then maybe that means that Walter and the other boys also did, and that maybe Walter is out there right now, alive but afraid to come forward, thinking that something might happen to him or to his mother. Whatever the case she says she now has something she didn't have before. Lester asks her what that is, and she looks him in the eye and simply says, "Hope".