‘True, but for every person in Whitehall who thinks we should be cosying up even closer to America there’s someone else who wants us building bridges with the Russians,’ Holland said. ‘And if they end up firing nuclear warheads at each other they’ll be flying over our heads.’

Britain’s own nuclear defence development had stalled after the de Havilland Blue Streak missile programme had been cancelled a year earlier. And no rocket with the Union Jack on its side would be shooting for the moon any time soon. This was one of the reasons Pipistrelle was so important—the intelligence it collected gave MI5 a much-needed edge in a rapidly changing world.

Or they’ll get us to do it for them,’ Knox said.

Desperate to make up for the UK’s shortfall, Macmillan had authorised the RAF to carry US nuclear warheads. The arrangement was supposed to be built on clear distinctions: the missiles were American, and the planes were British. But it was never certain whose finger would really be on the launch button. The idea of the UK being dragged into a nuclear war because of a trigger-happy US general or president made a lot of people nervous, including in Leconfield House.

‘Quite,’ Holland replied.

‘Which side do we pick?’ Knox asked.

‘Neither,’ Holland said. ‘We stay vigilant, and pay close attention to anyone inside or outside this building who might be trying to curry too much favour with either one. I don’t like the idea of surviving the battering of two world wars just to end up as the fifty-first state or a tinpot Soviet republic.’

That conversation had happened three years ago, and ever since Knox had been hunting moles. He’d been exhaustive in his quest, but had found little evidence of anyone working against MI5 from the inside, until a throwaway revelation in Sandra Horne’s testimony eventually led him to suspect that Manning, then an assistant director, was a Soviet agent.

While most of the information Sandra and Peter passed on to Moscow came personally from Montcalm, Peter would also travel into London once every two months to retrieve coded messages from a second-hand bookshop in Cecil Court. Knox had cross-referenced the times of these trips with Montcalm’s movements and established that he hadn’t been leaving the messages for Horne to collect. The question then was, who had?

Knox reviewed the records of the Service’s most senior members, looking for signs that would point to one of them being the guilty party. After several late nights reading through old reports called in from record storage, Knox had begun to appreciate some of White’s enthusiasm for the speed and efficiency offered by Atlas. But he still believed there was no substitute for studying the actual files, understanding their context and subtext, and reading the specific words that were used and—importantly—the ones that weren’t.