Fiction and non-fiction can feed off each other in unusual ways. In the winter of 2018, Germany was shaken by the biggest media scandal since the forged Hitler diaries, after it emerged that the country’s bastion of investigative journalism had published stories by a reporter who had “fictionalised” his prize-winning articles with armies of invented characters. Now the “Relotius scandal” has gone into its second round, with the publication of a non-fiction book by the journalist who exposed his fraudulent colleague: a detective story about the search for truth in the era of fake news that makes a more gripping read than most novelists could have managed.

The plot of Juan Moreno’s Tausend Zeilen Lüge (A Thousand Lines of Lies) resembles that of the 2003 film Shattered Glass, about New Republic reporter Stephen Glass – a film that the young Der Spiegel journalist Claas Relotius was shown as a cautionary tale as part of his journalism degree. Claas Relotius was regarded as a likable colleague with a great talent for unearthing incredible stories from across the globe: a vivid inside account from a high-security prison in California; the tale of how a boy’s graffiti started the war in Syria; an exclusive interview with the parents of American football star Colin Kaepernick.

But the scale of Relotius’s deception was more ambitious, and, for a while, more successful: before he was exposed, he had won the top prize for German reporters, the Reporterpreis, four times in six years. One judge praised his stories as jumping off the page “almost like literature”. He was in line for a promotion at Der Spiegel.

Juan Moreno. Photograph: Armando Babani/EPA

His rise and eventual fall also had a distinctly German note: many of the country’s high-profile publications have a hierarchy that places reporters with a literary flourish, known as Edelfedern or “classy quills”, at the very top, and stories in the back pages of a daily paper can start with a six-paragraph scene setter; some journalists complain that the culture prioritises storytelling over detective work. At Der Spiegel, long reads are often co-authored. In October 2018, Relotius was paired with the paper’s regular freelancer Moreno to write about the caravan of Central American migrants making their way to the US. It would prove his undoing. Moreno was to follow a group fleeing their country in search of a better life, while Relotius was planning to infiltrate a vigilante border patrol. The fact the 32-year-old journalist managed to achieve this within days, along with the script-like instructions he issued to his co-author, (“Let her just get onto a bus, into a trafficker’s car […] Like in a good film”) raised Moreno’s suspicions. Risking falling out with his editors, Moreno went on a two-month journey to expose his colleague’s lies, travelling to the Sonoran desert to meet the vigilantes whose backstories Relotius had so dramatically embellished.

What makes A Thousand Lines of Lies such a riveting read is that Moreno doesn’t present himself as an invisible arbiter of truth, but remains honest about his self doubts and the ego that drove his quest. The son of Andalusian migrants, Moreno writes how he sometimes feels like an impostor in the predominantly white world of German media. At Der Spiegel, he had found a niche “as a kind of expert on people with black hair”. Then came blond-haired Relotius, almost 15 years younger and a “high-performance Teuton”, kissed by the reporter gods.

With such antagonists, it’s little surprise the book has firmly established itself in the bestseller charts, and a film adaptation is scheduled for 2021. The book’s integrity has so far survived a post-publication post-script: in October, Relotius’s lawyer announced he was suing Moreno over 22 instances of “considerable untruths and misrepresentations”. In one instance, he alleged, the author had described a scene which had never taken place. Two months later, Moreno says he is still waiting for Relotius’s lawyer to follow through with his threat. “All the time while writing this book I was expecting to be sued over it,” he told the Guardian. “So why would I have made stuff up? That is insane.”Describing his experience on the borderlands of fact and fiction, in the book Moreno writes: “You can make sense of the world at your writing desk, but then you leave the office and realise nothing is as simple as you imagined it to be.”