The change was astonishing. A new person was speaking, in an indescribably sad voice, who didn’t need to remind Dyer how, in the cause of religion, the Ayatollahs had created a fascist state that rivalled the fascist states of 1930s Europe.

‘You wouldn’t believe the wickedness that is being done in the name of good – it’s big.’

Iran was supposed to be a country dedicated to God. It was not. Religious diversity was despised, said Marvar, his voice tightening. ‘I’m not a religious person, but my mother – she was a believer, and how she suffered because of it. How she suffered!’

Soon after his father’s death, when Marvar was seven, his mother had converted to Christianity. She experienced no repercussions at first. Then, five years ago, she was detained as she left the Janat-Abad church in Tehran, and her Christian books confiscated. Eventually released, she was reminded that apostasy was punishable by death. She died of pancreatic cancer eleven months later, still unable to worship freely.

What Marvar believed in was science. He seemed not to have considered his position when he began his research, the implications of success: he had been pursuing his goal purely for science’s sake. Now he had to confront the political realities of his discovery.

The intolerance shown to his mother had germinated an already rebellious seed. Marvar was a student at the University of Tehran when he came across Werner Heisenberg’s line that ‘effective resistance can only come from those who pretend to collaborate’. This impressed him, but less than the injunction of another atomic physicist, Fritz Houtermans: ‘Every decent man confronted by a totalitarian regime ought to have the pluck to commit high treason.’ In the liberating world of Oxford, exposed to scientists from every part of the globe, he had mustered the courage. He had resisted passing on his latest research to Tehran, despite the sharpening pressure on him from the Revolutionary Guard.

‘I have still not alerted them.’

Inevitably, the Guards and their Mullah bosses would view his behaviour as treasonous, but Marvar had worked tirelessly to brush over his tracks. He had locked his notes in a safe in Oxford, while, to the Iranian Embassy in London every Saturday, he continued to send routine reports and theoretical studies – ‘bromides, essentially’. He copied these in to his team at the Clarendon. Until the arrival of Ralph Cubbage, Marvar had managed to keep his Australian professor in the dark. In Professor Whitton’s eyes, Marvar was a diligent if unsociable junior spectrometrist who dabbled not terribly successfully with magnetic levitation. It would never have crossed Whitton’s distracted mind that this plodding thirty-nine-year-old Iranian researcher had made imminent the feasibility of cheap fusion, any more than it would have entered the thinking of Marvar’s masters in Tehran when they permitted him to go to Oxford.