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The policeman on the end of the line spoke calmly, clearly intent on catching the thief who had swiped a customer’s purse.

And as he described the suspect – a young brunette wearing a red tie – McDonald’s assistant manager Donna Summers knew exactly who it was, staff member Louise Ogborn who was just finishing her afternoon shift.

The shy, hard-working 18-year-old hadn’t given any problems since starting her job four months earlier.

But on the cop’s instructions Summers called her into the office, took her car keys and phone and locked the door.

The officer stayed on the line and told Summers that in order to preserve evidence she must instruct the girl to strip naked, put the clothes in a bag and take them away. The assistant manager did as she was told.

For three hours Summers – and later her fiancé Walter Nix – did everything “Officer Scott” asked, even as his demands became more bizarre and increasingly perverted.

But the voice on the line wasn’t a lawman at all. Terrified Louise was just the latest victim of one of the strangest and cruelest phone hoaxers ever.

The same caller – believed by cops to be 38-year-old prison warder David Stewart – is thought to have duped managers of more than 70 fast food outlets in 31 US states into strip-searching, humiliating and sexually abusing customers and staff.

By the time Summers realised she had been tricked, Louise had been made to dance naked with her hands above her head, do jumping jacks and deep knee bends, sit on Nix’s lap, kiss him, have her buttocks slapped until they were red, then perform oral sex on him.

New film Compliance, starring Dreama Walker and based on the sick 2004 hoax in Mount Washington, Kentucky, opened yesterday.

But anyone seeing it will be left asking: Why did they go along with it?

Louise said she co-operated because she couldn’t afford to lose her £4.20-an-hour job.

She later said she begged not to be strip searched, adding: “She was my manager and supposed to take care of me. I was petrified.

“My parents taught me when an adult tells you to do something that’s what you do. You don’t argue, you listen.

“Every time Donna came in the room I begged ‘Get me out of here. I didn’t do anything’. My soul just left my body and I went numb.”

In a lawsuit against McDonald’s she said: “I was bawling my eyes out and begging them to take me to the police station.”

But Summers said the caller sounded so genuine she was happy to follow his instructions. “I did exactly what he said,” she later explained.

“When I asked him why he always had an answer. I honestly thought he was a police officer.”

Scarily, another manager and two employees saw the abuse but did nothing – believing that because police ordered it there was nothing wrong.

“Officer Scott” even convinced Summers to call fiancé Nix, 42, to take over the “punishment”.

He eventually forced the teenager – on the caller’s orders – to perform oral sex on him.

After leaving the restaurant he immediately called his best friend and told him: “I have done something terribly bad.”

Two and a half hours later Thomas Simms, a 58-year-old maintenance worker, refused to take over the abuse and made Summers call her area manager.

She later said: “I knew then I had been had. I lost it. I begged Louise for forgiveness. I was hysterical.”

When she saw the CCTV of what Nix had done she called off the engagement.

He was jailed for five years for sexual abuse. Summers was sacked and got a year’s probation for false imprisonment.

Louise, who now has a five-year-old daughter, sued McDonald’s for £127million but later accepted £700,000 out of court.

A month before her ordeal, a caller claiming to be a cop persuaded the 16-year-old female manager of a Missouri drive-in restaurant to strip search a 21-year-old cook accused of stealing a purse then perform oral sex on him.

The man then asked for the phone to be passed to the cook. He told him the young manager was the real suspect and convinced him to strip search her too.

A month before that a manager at a Taco Bell in Arizona also claimed he was following phone instructions after strip search-ing a 17-year-old girl customer and making her exercise naked. Retired FBI agent Dan Jablonski, who investigated the cases, said the caller exploited the common fear of “the cop in your rear view mirror”.

He said: “You are stopped and your first thought is you’ve done something wrong so your reaction is to be compliant.”

He added that fast-food staff made easy targets. “You and I can say they were blooming idiots. But they aren’t trained to use common sense. They are trained to say and think ‘Can I help you?’”

In all, seven people who executed strip searches at the fake officer’s behest were convicted and jailed.

But dad-of-five David Stewart, the man police believe was behind the calls, was acquitted in 2006 of impersonating a police officer, solicitation to commit sex abuse and unlawful imprisonment because of lack of evidence.

It was only after Louise went to police that the pranksters’s alleged 10-year spree of deception began to unravel.

Mount Washington policeman Buddy Stump discovered the call that began Louise’s ordeal was made from Florida with a pre-paid calling card.

(Image: Google)

When he called the WalMart store where it was bought he discovered another officer was also investigating.

After studying CCTV the pair arrested prison officer Stewart who had been turned down for dozens of police jobs.

He denied buying the calling cards but at his house police found one which had been used to call nine restaurants.

Since Stewart’s trial reports of hoaxes have all but stopped – but not entirely.

In March 2009 a caller talked the manager of a New Hampshire KFC into activating the sprinklers, raining fire retardant down on her and staff.

The hoaxer then convinced them all to go into the car park, strip and urinate on one another to neutralise the chemicals.

Those appalled by these stories have been left wondering: What would I do?

But as Walter Nix’s lawyer Kathleen Schmidt said at his trial: “I can understand how people can question what he did. But they weren’t in the same position.

“Maybe you and I wouldn’t have done this. But how do we know?”