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Every year, thousands of Britons vanish from their lives. Kirsty and Zack had just celebrated the birth of their son when Zack suddenly walked out of their home. This is the story of how Kirsty tried to track him down.

It's breakfast time and Zack, 28, is running late for his job in a High Street bank. His partner, 34-year-old Kirsty, tells him to hurry up, but he's laughing. He gives Kirsty a kiss and says goodbye to the children - a seven-year-old and a baby of just five weeks. As he walks out to the street, the eldest child goes to the window and makes heart shapes with her hands to Zack below. It's an ordinary morning.

A couple of hours later, Kirsty receives a text message from Zack to say he's not coming back.

The morning he disappears, Zack phones his work to say he can't come in. Instead, he walks into a police station and tells them if anyone reports him missing, he doesn't want to be found.

Kirsty discovers this when she fills in a missing person form. It's immediately sent out to police stations around the country. A few minutes later, Kirsty is told that Zack is, in fact, alive and well but doesn't want her to know where he is.

"It made me cry because it was so cold," says Kirsty.

Find out more

Kirsty told her story to producer Georgia Catt on BBC Radio 4's The Untold. Kirsty's story can be heard here and here on iPlayer.

She's particularly worried because Zack has been diagnosed as bipolar.

"I knew he wasn't well," she says. But he would have appeared in control when the police saw him.

When Zack walked out of their flat, he took nothing with him - including the medicine he'd been prescribed. Kirsty knows that without it he'll suffer cold sweats and depression.

At this point, her thoughts are swinging wildly from worrying about Zack's health to wondering if he just walked out on them all.

Is he ill? Or has he done a runner?

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The couple have been together for five years. They first met at university, where Kirsty remembers Zack as a brilliant student who spent his spare time writing plays.

Kirsty's eldest child, her seven-year old daughter, is from another relationship. Before their son was born, they had all been staying with her parents, but had recently found a flat to move into. Zack had only been in the new flat for two nights before he disappeared.

Their home is full of piles of books scattered around the floor, waiting to be put up on shelves.

But Kirsty's mind is preoccupied with her search for Zack.

She makes a missing poster with his photograph and contacts local doctors to see if he has registered anywhere. She deposits small amounts of money into his bank account - just so she can write short messages in the 15-character reference field. She feels she's clutching at straws.

Most of all, she's concerned about his health.

"If he's in a little studio apartment somewhere and he's done himself in, who actually is going to find him?" she asks.

Zack has now been missing for 34 days.

Kirsty still has no idea where he is, but a routine call to the Child Support Agency leaves her wondering if he's ill.

To her surprise, she's told they have a reliable address for Zack.

Her first feeling is shock as she realises her partner is probably safe and well. And then frustration sinks in - everyone else seems to have access to Zack's address and new life apart from her.

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"I thought we were planning on this future together," she says.

"It didn't involve him doing a runner. I feel cheated. He's left me with all this responsibility."

Then, to add to her sense of alienation, she discovers several email accounts Zack has set up using both their names together.

"Is this a game of cat and mouse?" she wonders. "Is he getting a little bit of sick pleasure knowing that he is traceable, but it's all very vague?"

She's able to hack into one of the accounts because Zack has used a password she knows. Inside she finds an email that includes part of a postcode which she tracks down to a village outside Nottingham.

Kirsty believes her husband is renting a cheap flat there and thinks about confronting him.

Zack has now been missing for 41 days.

Kirsty discusses her plan to go to Nottingham with her brother. He tries to talk her out of it.

"We just don't know why he left or anything that was going on in his head," he says.

About six months ago, Kirsty noticed money was going missing from Zack's account. He admitted he'd been gambling online. Kirsty's brother thinks he may have an addiction problem - something Kirsty finds hard to believe.

"I think he was gambling and you've got a new kid and the rent was due and he was too scared to explain to you he didn't have it, so he ran away," says her brother.

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But Kirsty's mind is made up. It takes her eight hours to reach the village, driving through heavy traffic with her baby in the back seat of her car.

She's booked a room in a hotel on the road where she thinks Zack is living. And she's made a flyer with pictures of her and Zack, and Zack with her daughter - Zack as a family man - which she plans to distribute on the road.

She admits she included pictures of herself and her children just in case Zack is with another woman - she wants that woman to know they exist. The flyer also has a personal message on.

"I've said it's the hardest letter I've ever had to write, and I've only got one shot at getting it right," says Kirsty.

"I've said I'm staying up the road in this hotel until tomorrow afternoon and I'd really like to see him."

The next morning, there's a man waiting to see her in the hotel lobby. For a moment, Kirsty thinks it could be Zack. But it's a stranger who tells her he's read her flyer and hasn't seen her partner. He just wanted to let her know so she wouldn't hold out false hope.

Kirsty returns home.

"I just think I've got to not expect to hear anything for a long time," she says.

Zack has now been missing for 45 days.

Suddenly Kirsty receives a message from the NHS advice service, 111.

A woman working there says she has received a call from a man in the city of Bath who said he could only remember his name and age. The woman Googled this information and saw that Kirsty was searching for him.

This seems like the first concrete lead. But if it is Zack, he appears to be suffering from amnesia.

Kirsty decides to take this information to the police.

She's armed with further confirmation that Zack is in Bath. After posting a message on Facebook, she hears from two Bath-based hairdressers who say they are convinced that Zack was in their salon.

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But her meeting with the police doesn't go well and Kirsty doesn't think they believe her.

"They look at you as though you've got no right to be here," she says.

And then the station sergeant says he wants to speak to her in private.

He tells her that Zack has been in touch. Police officers went to see him this morning and he told them he doesn't want Kirsty to know where he is. He says he's not ready to come back.

Kirsty is devastated. She doesn't understand why Zack would phone 111 and say he's confused, but then tell the police he's ok.

At the back of her mind is the thought that she must have been a really bad girlfriend. This is Zack's first serious relationship and she wonders whether she forced too much responsibility on him too early. When they first got together, her little girl was still a baby so they couldn't do the things that new couples often do - enjoy lie-ins, go away for the weekend.

But at the same time, she remembers what it was like when Zack last experienced a bipolar episode. Then he said he wanted to hurt her because she was so good. She wonders if he's trying to hurt her now.

Then, that night, Kirsty gets a text message from an unknown number.

Whoever it is knows about the messages she's been leaving in Zack's bank account. This person says they're confused and they're planning to go to the police. They say they'll call Kirsty straight afterwards. It's 11pm. Kirsty doesn't hear from them again.

She's convinced the person is Zack, but she doesn't believe he has amnesia.

The next night she receives a phone call.

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For a couple of seconds there's silence, then the caller hangs up. The phone rings again and it's Zack. Finally Kirsty has some answers.

During their hour-and-a-half conversation Zack says he's trying to remember things. He knows his name and age but only vaguely remembers Kirsty. They agree that Kirsty will pick him up the next day.

But Kirsty doesn't hear from him again.

Christmas comes and goes without Zack.

Kirsty keeps checking his email accounts and sees that he has been using online gambling sites frequently.

He's been placing small bets of £5 and £6, but he's been placing a lot of them, sometimes 20 a day.

Then suddenly she sees that Zack has started betting £500 in one day.

Zack has now been missing for 69 days.

Out of the blue, Kirsty receives a call from an unknown doctor saying Zack has been with him for the past 10 days. Zack was placed in his care by an outreach charity who picked him up off the streets.

"I'm so relieved tonight to know that people are looking after him now," she says.

Kirsty finds out Zack has been in Bath, and then Hertfordshire, and Essex. There's no mention of Nottingham. Some of the time he's been working and paying for accommodation. Other times he's been loitering, unclear of where to go. The doctor tells her that he's very unwell, flipping between personalities and he genuinely can't remember things.

"He wasn't lying," says Kirsty, relieved.

Finally she has a number she can call Zack on. When they talk he asks her lots of questions about how they met and what their life was like.

Zack describes how he has been wandering around following a voice inside his head that said it could take him home. But each time he followed it, he became more lost.

Then, 76 days after he walked out of her life, Kirsty is told that Zack is well enough to go home.

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A month later, Kirsty is still getting used to having Zack back. She believes the reason he left was because of the upheaval of having a new baby and moving house at the same time.

"It's all really hazy," says Zack.

"It was Bath first of all, then it was Canada Water [in London], then after that Essex. I know that things would have been going on, but I don't remember them and it's all really grey."

He does remember bits of being away - going to the police station, and being on a bus - but feels disconnected from himself.

At one point, he remembers living outside in the woods with nothing but a coat to protect him.

He describes the experience as being "split". It was as if he was standing outside himself looking in. Especially when he was making bets with huge sums of money.

"I would have made lots of plans," he says. "But shortly after leaving, I found myself homeless. So how planned was it?"

Where to get help

If you have been affected by the disappearance of a loved one, the following charities can offer support and advice:

Missing People - a charity dedicated to bringing missing children and adults back together with their families.

The Samaritans - offer a safe place to talk about issues at any time.

UK Missing Persons Bureau - the UK national and international point of contact for all missing person and unidentified body investigations.

Even though Zack still suffers from what he describes as a weird energy in his mind, he says he can distract himself from it. Being back on medication has helped.

"I'm seeing doctors and I'm seeing a psychiatrist who's very good. My concern now is being a good dad and a good partner," he says.

But Kirsty and Zack's relationship remains fragile. She says she's worried he might leave again and if he does, this time she won't try to find him. She finds it hard to trust him, especially after reading the emails in his secret accounts which she was clearly not meant to see.

One of the saddest things is how some of her family and friends are still suspicious of her partner. Her best friend recently unfriended her on Facebook because she said she couldn't bear to see pictures of Zack and Kirsty looking happy together after all Kirsty had been through.

Kirsty doesn't know if her relationship will work out.

"One thing I'm really certain of is this won't happen again. He wouldn't just go," she says.

"But he might start gambling, he might start lying… that's something I can't control, but also can't have around me."

All names have been changed.

Listen again to Kirsty's story here and here on iPlayer.

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