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At first, the story is heartbreakingly familiar. A single mother returns home from work only to find her 9-year-old son has vanished. She panics. Was he kidnapped? Is he lost? Initially the police are no help. Then six months later they miraculously find the boy halfway across the country. That's when things get strange: The boy is scrawnier and shorter than the one who disappeared. The mother's first words upon what should have been an emotional reunion: "I do not think that is my son."

If the case of Christine Collins and her missing son Walter had happened last week, it would be topic one on Nancy Grace. Instead it was a media sensation in 1928 and is now the subject of Changeling, the new movie directed by Clint Eastwood and starring Angelina Jolie as Collins, whose belief that the child returned to her was not her own led to her being committed to a psychiatric ward.

"Here was someone who really put her life on the line in one of the most bizarre and courageous stories of L.A. history," says screenwriter J. Michael Straczynski of Collins, who later won a lawsuit against the officer who had committed her. "She never wanted to be a hero, she just wanted her son back." (Spoiler alert: The rest of this article reveals the ending of Changeling.)

Sadly, Collins never saw her son again. The boy who claimed to be Walter was in truth Arthur J. Hutchins, an 11-year-old Iowa-born runaway whose fervent desire to get as far away as possible from a stepmother he felt didn't love him was matched only by his ability to fool the police, Walter's friends and even Walter's dog and cat into thinking that he was Walter. "Tootie, the cat, didn't like anyone but Walter, but she made friends with me," Hutchins wrote four years later in a 25-page recounting of the incident, shared exclusively with PEOPLE by his family. "Even Tiny the dog acted as if he knew me. That helped convince Mrs. Collins."

But in her heart Collins knew this boy was not Walter—even as the police, under pressure to close the case, tried to explain his smaller frame as the effects of torture by kidnappers. Collins' conviction was affirmed when she took the boy to the beach. "When I got my bathing suit on, I ran like a streak to the water," wrote Hutchins, who died at 35 in 1952. "The boy I was posing to be could not swim; in fact he was afraid of water. Right then Mrs. Collins was most sure I was not her son."

Collins was in the psychiatric ward for more than a week, during which Hutchins finally confessed. So what happened to the real Walter Collins? He was probably a victim of Gordon Stewart Northcott, a pedophile and serial killer who was convicted and hanged in 1930 for the murder of three boys and is believed to have killed many more. Northcott destroyed most evidence of his crimes, but hair strands closely matching Walter's were found on the ranch where the murders took place, and Northcott's nephew identified Walter as a victim. "My guess is that he did die up at the ranch, but he may have gotten away," says Straczynski. "We will never know for sure, but we do know that his mother never lost hope."