For more than 20 years the Prince and Princess von Hessen saw themselves as a happy “McFamily”, toiling away in their four McDonald’s franchise restaurants like something out of a social democratic fable.

The pair had first met in the chain’s Munich branch and their four children practically grew up in its aprons and paper caps, with “ketchup in their blood”. The family kept at it despite the couple’s divorce and the prince’s death in a road accident two years ago.

“I really did everything: the kitchen, the tills, the drive-through, the cleaning at night,” said Carla, Princess von Hessen, who inherited the business after her former husband died.

Yet the idyll has curdled into an increasingly bizarre legal dispute as McDonald’s attempts to seize control over the lucrative franchises in the medieval Bavarian university city of Ingolstadt.

Von Hessen, 48, accuses the fast food chain of stooping to a “McMafia” campaign of bullying and harassment, with hired goons, private detectives, mysterious men snooping through her family’s wastepaper and spurious inspections by the local public health agency.

McDonald’s, for its part, insists she is not fit and proper to run its restaurants, and has dismissed her claims as lacking “any factual basis whatsoever”. The two irreconcilable sides of the argument have only one thing in common: neither of them, it seems, is loving it.

The Von Hessen family are descended from a minor branch of what used to be one of the most powerful of Germany’s royal dynasties. Since the German aristocracy was formally abolished with the 1919 Weimar constitution, the “prince” and “princess” titles are strictly part of their surname.

As a young man Otto von Hessen did his training at McDonald’s and duly inducted his wife into the family trade after they married in 1994. Over time they built up a modest empire of franchises across the city, whose combined value has been estimated at between $3.7 million and more than $12.3 million.

The morning after Otto von Hessen died and left it all to his ex-wife, McDonald’s rang her and said they needed to talk. She did not realize anything was amiss until a month later.

“McDonald’s sent an email to my senior staff and organized a meeting in the morning … without informing me,” she said. “Then they told my senior staff that no one from the Hessen family, regardless of whether they were the heirs, would take over the restaurants. The raid came as a total surprise.”

It was the first of many. McDonald’s maintained that Von Hessen was unsuitable to operate the franchises because she was impulsive and had little relevant experience, having spent most of her time in a local boutique instead. Von Hessen said this was nonsense: “I trained the staff. I did all the accounts. It felt like being in the branches 24/7.”

She sued for control of the franchises, but lost the first round at a regional court in Munich. Her appeal is expected to be heard this year.

In the meantime the case has taken a darker turn. “Of course, I can only conjecture [about this],” Von Hessen said. “But one evening I come home from Ingolstadt: I park my car, I get out, close the door and lock it. And there is a fully tattooed muscleman standing in front of me who tells me it would be better if I just left things alone now, because you never know what might happen.

They built up a modest empire of franchises across the city, whose combined value has been estimated at between $3.7 million and more than $12.3 million.

“Apart [from McDonald’s] I don’t have any enemies who would do something like that to me. Another time I go out of the house to take the rubbish out — I always get up very early because I have four children — and I see that someone has gone through my wastepaper bin.”

Von Hessen has resorted to theatrics of her own. In February she turned up at one of the branches flanked by 12 men in blazers to symbolically reassert her rights to the franchise. She has also claimed that McDonald’s is using underhand financial tricks to “bleed her dry” in the hope of driving her into insolvency so that the contract is voided.

“I really thought before that we were a McFamily and we’d sit around the table and talk constructively, and address everything professionally,” she said.

“But what is happening here is anything but nice and pleasant … I’ll put it this way: you always have your downs every now and then. But I also expect a positive outcome, because I simply believe that this is not how you treat widows and orphans, and that what McDonald’s keeps doing to me every day is without any class or decency. It is manipulation and intimidation. But as I always say, karma will come around.”

The German arm of McDonald’s denied wrongdoing and said it had repeatedly sought a negotiated settlement with Von Hessen. “The allegations … have no factual basis whatsoever,” it said. It added: “The four contested restaurants in Ingolstadt belong to McDonald’s Germany, as was confirmed by the judge in the first court verdict on the case. Carla, Princess of Hessen, in particular has no right under the franchise contracts to intervene in the business operations of the McDonald’s restaurants in Germany.”

Oliver Moody is a Berlin-based journalist for The Times of London and co-host of the podcast Tommies & Jerries