When the police called Raphaël Halet, he was at the doctor's office; his back was acting up again. On the phone, a French official informed Halet that someone had broken into his home and stolen one of his cars. Could he please come immediately? Halet took off at once, never even thinking it might be a trap.

When he arrived at his house in the tiny village of Viviers in France's Lorraine region, there were several people waiting for him, including two police officers, a bailiff, a locksmith, a computer specialist and a representative of his employer, the Luxembourg branch of the international consulting firm PricewaterhouseCoopers (PwC). There hadn't been a break-in, nor had one of his cars been stolen; that had all been just a story to lure Halet back home. Once there, he was presented with a five-page warrant that had been issued the day before by a judge in Metz, France. It was a remarkable document. In it, the French government had taken the side of a billion-euro multinational corporation keen on finding and punishing a whistleblower within its ranks.

Raphaël Halet was that whistleblower, and the documents he leaked to a journalist formed the basis for the Luxembourg Leaks scandal that broke a few weeks before the house raid in Viviers. A consortium of European media, including the Süddeutsche Zeitung (SZ), analyzed the internal documents and published them in late 2014. Thanks to Halet, it was revealed that PwC had been a key accomplice in helping corporations avoid millions of euros in taxes by way of Luxembourg – with some of the methods used later being found illegal. PwC, for its part, claimed that it had always acted in accordance with the law. The scandal itself damaged Luxembourg's reputation as a financial hub and carried some political backlash for the country's former prime minister, Jean-Claude Juncker, who had just been elected president of the European Commission. Juncker is regarded as one of the architects of Luxembourg's tax avoidance scheme. But there was one party that didn't feel any backlash whatsoever: PwC.

In the search warrant, the judge had permitted investigators to copy Halet's emails, "including any email, and any part and attachment, addressed to or received from a journalist." Halet later told journalists at a meeting in Metz that the bailiff and the PwC representative had summoned him into his office, where they had him turn on his computer and sign into his email. The investigators were looking for emails to or from a single person: Edouard Perrin, a French investigative journalist. And they found what they were looking for: The whistleblower and the journalist were taken to court.

All this occured at the behest of a corporation with which most people are largely unfamiliar. Whether by its acronym, PwC, or its full name, PricewaterhouseCoopers, the consulting firm doesn't offer any services that ordinary people would buy or use and many people know nothing about it, even though PwC has hundreds of branches around the world, 21 of them in Germany alone. Even people who have heard of PwC probably couldn't say exactly what the company does. Which is why most people aren't particularly concerned about it.

But they probably should be, as joint reporting by the SZ and the two German broadcasters NDR and WDR has shown. PwC is one of the most powerful companies in the world, especially when it's in lockstep with the other three big global consulting and auditing firms, KPMG, EY (formerly known as Ernst & Young) and Deloitte. Known together as the "Big Four," this quartet of companies generally flies under the radar of public awareness. They are faceless, but together they represent a sinister power with a global reach.

Even if clients get punished, their quiet accomplices escape unscathed

In recent years, these four companies have repeatedly played key roles in a spate of major scandals, including some that have been investigated by the SZ, such as the Luxembourg Leaks and the Panama Papers. But even when the Big Four were accessories to partly illegal or at least illegitimate acts, they have always managed to walk away unscathed. Even when their clients are publicly denounced or pilloried, hardly anything ever sticks to the Big Four. One of the most internationally renowned experts on the Big Four is also one of their loudest critics: the British economics professor Prem Sikka. Rather than using the term Big Four, he refers to them as "the pinstripe mafia."

Its an evening in autumn 2018 and Sikka is just leaving PwC's London headquarters near the Embankment tube station. The building is an imposing structure of concrete and marble that sits on the bank of the River Thames. In Britain, the operations of the Big Four are more widely debated than they are in Germany – which is part of the reason the Labour Party commissioned Sikka to conduct a study on possible reforms of the Big Four. Indeed, that is the reason for Sikka's visit to PwC headquarters this evening. The visit, though, wasn't overly pleasant for either party, according to Sikka's account. The professor makes a point of referring to the Big Four by the pejorative nickname "pinstripe mafia" and he openly argues that they present a danger to democracy. "This mafia doesn't shoot people, but its activities are just as deadly," he wrote in a journal article several yeas ago. "They deprive millions of jobs, education, savings, pensions, security, food, healthcare, clean water and social infrastructure."

These are serious accusations. To understand them it is first necessary to explain what it is the Big Four actually do: PwC, KPMG, Ernst & Young and Deloitte offer consulting services to large corporations, banks and the super-rich in more than 180 countries. Together they have more than a million employees who advise clients on matters pertaining to tax structures, business strategy and a wide variety of other fields. Sometimes they prescribe sweeping restructuring, at others a reorientation, and at still others, they develop tax avoidance models. The Big Four earn the vast majority of their income through consulting fees.

In addition, the Big Four audit financial statements and balance sheets on behalf of their clients. These audit reports are presented to supervisory boards, investors and authorities and are supposed to show a company's financial health and indicate where improvements can be made. The Big Four have divided the international auditing market amongst themselves, at least when it comes to the largest companies; 99 of of the 100 companies listed on the Nasdaq 100 stock market index are Big Four clients.