It's 1928 Los Angeles and Christine Collins (ANGELINA JOLIE) is a single mom and phone operator supervisor who's raising her 9-year-old son, Walter (GATTLIN GRIFFITH), by herself. After filling in for someone at work, she returns home in the evening to discover that her son is missing. Frantic, she calls the police who say they can't do anything until the boy has been missing for 24 hours.
Days and then months later, Walter is still missing, with Christine taking every break she gets from boss Ben Harris (FRANK WOOD) to call authorities around the country asking if they've come across anyone matching Walter's description.
Her plight has also drawn the attention of Rev. Gustav Briegleb (JOHN MALKOVICH) who routinely uses his radio broadcasts to blast the Los Angeles police department -- and specifically Chief James E. Davis (COLM FEORE) -- for its violent tactics, intimidation and general corruption, and sees their failure to find the boy as yet another example of their incompetence. Yet, the LAPD seems to get a positive PR spin on news that Walter has been found in another state and that Capt. J.J. Jones (JEFFREY DONOVAN) will be delivering him to Christine.
The only problem is that she knows deep in her heart that the boy (DEVON CONTI) isn't her son, despite Jones assuring her he is -- stating Walter has simply changed in appearance over the intervening months -- and the boy even knowing his home address. Even so, she's psychologically bullied into taking the boy home where Dr. Earl Tarr (PETER GERETY) -- working for the LAPD -- confirms that the boy is Walter despite having never met him before.
When Christine continues to complain, including publically with the help of Gustav, Jones -- fearing a public backlash -- has her committed to the psychopathic ward under the supervision of Dr. Jonathan Steele (DENIS O'HARE). There, she learns from fellow inmate Carol Dexter (AMY RYAN) that she's not alone in being hidden away by the cops and that no matter what she says, the staff will view her as mentally incompetent.
At the same time, Det. Lester Ybarra (MICHAEL KELLY), working the missing children beat, retrieves young teenager Sanford Clark (EDDIE ALDERSON) from a desert ranch and soon hears horrific tales about his guardian, Gordon Northcott (JASON BUTLER HARNER), a bloodthirsty child abductor. That revelation, coupled with Gustav enlisting the aide of powerful attorney S.S. Hahn (GEOFF PIERSON) to represent her plight, ends up providing Christine with both fear and hope regarding finding what's really happened to her son.