Mary disappeared in 1981. When she 'returned' decades later, her sister knew something wasn't right.

Mary Louise Day was a sweet girl with a troubled past. By 13, she'd already endured more trauma than most people face in a lifetime.

Growing up in Little Falls — a picturesque town in New York's Mohawk Valley — Mary and her sisters, Sherrie and Kathy, shared a childhood that was anything but picture-perfect.

The girls were taken from an unstable home and bounced between foster homes until eventually, Mary was reunited with her birth mother, Charlotte, who had married soldier William Houle and settled in Seaside, California.

But this wasn't the happily-ever-after they'd dreamed of. In July 1981, Mary disappeared, seemingly without a trace.

Her sisters were told she ran away. Others believed she was murdered by her stepfather. With no body and no clues, Mary's disappearance became one of those mysteries that keep detectives awake at night.

Until November 25, 2003.

More than 1,100 kilometres away, a woman surfaced with Mary's exact name and date of birth.

It seemed impossible. Too good to be true.

Could this really be the same Mary Louise Day who'd vanished all those years ago? And if it was — where on earth had she been for the past two decades?

To understand what really happened to Mary, we need to go back to the beginning.

Watch the trailer for Investigation Discovery's new docuseries The Curious Case Of… The Girl Who Died Twice. Post continues below.

Who was Mary Louise Day?

On February 19, 1968, Mary Louise Day was born to Charlotte and Charles Day in Little Falls, a beautiful small city along the Erie Canal in New York. The couple later had two more daughters, Kathy and Sherrie.

The girls' upbringing was far from smooth sailing. They spent most of their childhood in and out of foster care.

Meanwhile, Charlotte had an affair with a soldier named William Houle, who went on to become Mary's stepfather after she left Charles. Once the couple got married, they were able to obtain full custody of Mary and Kathy, who were brought back to the family home.

Sherrie told Investigation Discovery's new docuseries The Curious Case Of… The Girl Who Died Twice, that she "hated" her birth mother and refused to leave her foster family. They later went on to adopt her, and she was free of the family home.

With Sherrie staying behind, the family followed William and moved around depending on where he was stationed. Together, Charlotte and William gave birth to two children, Billie Jeanne and William Jr.

But life back with her family didn't get any easier for Mary. Her stepfather was allegedly abusing her, and she repeatedly ran away to escape. Yet, police would always bring her back, according to CBS News 48 Hours.

In 1980, she was placed into protective custody due to her stepfather. But eventually, she was returned to the family — a move that may have cost Mary her life.

Everything changed in July 1981 when Mary vanished.

Mary Louise Day, front left, with her family. Image: CBS News.

The disappearance of Mary Louise Day.

In a shocking twist that even police couldn't wrap their heads around, Charlotte and William didn't report Mary missing… for years.

Eventually, estranged sister Sherrie received a letter from Kathy, who was still living with their mother Charlotte. The letter had a New York address on it, so Sherrie went to visit.

"I asked them, what had happened to my sister, Mary?" she told 48 Hours.

And the response was chilling. When she asked her mum for pictures of Mary, Charlotte said she "burned them".

"Kathy… was like, 'Shh, don't say anything. We're not allowed to talk about Mary'," Sherrie told 48 Hours.

But Kathy did share one piece of information. Mary had apparently run away. Sherrie wasn't sure what to believe. She knew it didn't make sense, but why would they lie?

It wasn't until 1992 that Sherrie reported the case to the police, officially filing a missing persons report.

By the time Seaside Police finally launched their investigation in 2002, there was little to go on.

Mary with her stepfather, William Houle. Image: CBS News.

Detectives brought Kathy back to their Seaside home in March 2003, and made her revisit the last day she saw her sister.

"That day lives in my head a lot. It feels like you're opening a scab, you're opening it up and it hurts," Kathy told 48 Hours.

She remembers being at home with Mary while the rest of the family was out. While Charlotte and William were gone, the dog became sick and was dying in the kitchen. Once William saw, he flew into a rage, immediately accusing Mary of poisoning the dog.

"He started yelling at us… and I got scared… all hell broke loose," Kathy said.

She saw William hitting her sister. Mary was just 13 years old.

The last time Kathy saw her sister alive, blood was running from Mary's mouth.

The next morning, their mother simply said Mary had 'run away'."

After Mary disappeared, Kathy said her parents ordered the children to stay away from one particular area of the back garden. When investigators dug it up, cadaver dogs found a little girl's shoe and a belt — but no body, according to 48 Hours.

Watch: Is she really Mary? ID's The Curious Case of... Post continues below.

Was Mary Louise Day murdered?

Police were sure a body had been buried when the cadaver dogs alerted, with their dog handler suggesting the body had been moved.

At this point, police had no doubt Mary's parents were suspects in her possible homicide.

During his police interview, William recalled the night Mary disappeared.

"Charlotte told me that that night she saw Satan in my eyes. And she said I was possessed by a demon," he said, according to CBS News' 48 Hours investigation.

When asked whether the "demon" inside him could have killed her, William said yes.

In Charlotte's interview with police, she said: "You know life is full of regrets. If you go back and say, you know, 'if I had did this, and this and this'."

Despite their statements, the couple has always maintained Mary ran away. But police believed they had enough to press charges… until they made a discovery that would shake investigators to their core.

A young Mary Louise Day. Image: ID.

An unexpected twist.

On November 25, 2003, more than 1,100 kilometres away in Phoenix, Arizona, a routine traffic stop changed everything. Police pulled over a truck with stolen plates, and one of the passengers handed over an ID. The name on it? Mary Louise Day.

The same name. The same birth date. It seemed impossible.

Initially, police were sceptical. When they questioned 'Phoenix Mary', she claimed to have run away from home to escape the abuse. However, she couldn't recall key details from her childhood and her family.

Police took a DNA sample from her and Charlotte. Shockingly, it was a match. But unfortunately this didn't prove she really was Mary, just that she was Charlotte's daughter.

Sherrie invited 'Phoenix Mary' to stay at her home but was unconvinced it was her sister. She described the woman as "scary" and told The Curious Case Of…. she thought she was crazy.

Investigators believed that Charlotte had other children they did not know about, and she convinced 'Phoenix Mary' to take on Mary's identity to avoid facing homicide charges.

Another theory was Mary had multiple personality disorder from the trauma, causing holes in her memory.

The girl who died twice.

In 2017, Seaside chief of police Judy Veloz got a call from a woman claiming to be the younger sister of Mary Louise Day.

The woman said 'Phoenix Mary' was in hospice following a cancer diagnosis. When questioned by police, she recalled running away from home but admitted she was still unsure about the details of the case.

'Phoenix Mary' died nine days later.

Phoenix Mary. Image: ID.

Today, the truth about what really happened to Mary Louise Day remains one of those mysteries that haunts everyone involved.

Charlotte and William refused an interview for the docuseries and have always maintained their innocence. They were never charged in Mary's disappearance.

For Sherrie, the questions continue to pile up, while the answers remain frustratingly out of reach.

What really happened that night in July 1981? Was 'Phoenix Mary' truly her long-lost sister? And perhaps most chillingly — will anyone ever know the whole truth about the girl who disappeared twice?

Feature image: CBS News.