On a cold December day in 2007, José Luis Peñas received a phone call from the man he had recently betrayed. Francisco Correa, a powerful business magnate, was ringing to ask whether they could meet the following evening. Peñas, a town councillor in a Madrid suburb, had worked with Correa for two years: they had started a political party together, to compete in local elections on an anti-corruption ticket. Peñas ran the campaign, Correa financed it. The pair were very different – Peñas was an affable bear of a man, while Correa, more than a decade older, was wiry and quick to anger – but they became close, talking almost every day, sharing confidences, dining with one another’s families. Correa’s young daughter even called Peñas Tio Pepe, Uncle Pepe. But within a few months, Peñas had realised that his friend was corrupt: Correa’s real business was conspiring with local politicians to rig lucrative public contracts. Instead of confronting him – or turning him in – Peñas spent more than a year covertly gathering evidence against his boss. Now, after amassing hours of secret tapes, Peñas had finally gone to the police to report Correa for a series of crimes that…