Isabel dos Santos’s secret to becoming a billionaire in Angola, one of the world’s poorest countries, was simple. She got her dad, José Eduardo dos Santos, Angola’s president, to give her money from the nation’s treasury.

O.K., it wasn’t quite that simple. A maze of shell companies, overseas tax havens and complex management and investment schemes funneled millions to and fro before the money ended up in her hands, according to an examination of more than 715,000 leaked emails, contracts and other documents obtained and examined by the International Consortium of Investigative Journalists and shared with several news outlets, including The New York Times.

Yes, this is another tragic tale of a resource-rich, underdeveloped country plundered by predatory political leaders and their families. But thanks to the investigation, we also know that Ms. dos Santos had the assistance of Western firms, including the Boston Consulting Group, McKinsey & Company and the accountants at PwC, who helped manage some of the 400 companies and subsidiaries in 41 countries controlled by Ms. dos Santos and her husband. In some cases, the couple’s advisers oversaw financial transfers that the current Angolan government describes as acts of money laundering. All the while, these Western advisers collected a share of the proceeds.

Boston Consulting Group “managed” a Swiss jewelry company that the Angolan government bought with money borrowed at a high rate of interest from a bank controlled by Ms. dos Santos. After she and her husband ran the jewelry company into the ground with lavish spending, the government was left owing $225 million.