‘You think Ian could have tipped off the PM, or someone close to him?’

‘I’m starting to think Ian is capable of almost anything.’

Kate bit her lip as she turned this over in her mind. Blaming Ian was reasonable – his appetite for promotion was voracious – but also, perhaps, too easy. ‘It doesn’t make any sense, though. If they were really trying to stage a coup, why back off now?’

‘Because they know we’re on to them,’ Sir Alan said. ‘Because they’ve lost the element of surprise. Because, after the car crash that was the attempted coup in Montenegro, they don’t want to look clumsy and amateur again.’

Kate glanced at Julie and Danny. Both were transfixed by the screens in front of them. ‘Or because the whole episode was designed to convince us of Mikhail’s good intentions,’ she said. ‘They stage an effective but brief theatrical show, which they know very well will be marked down as a quick win for us.’

‘A train of thought that leads us to a still darker and more confusing place, because, if they’re in a position to do that, it hardly suggests that Igor and his son have one foot in a Siberian gulag already.’

Kate didn’t know what to say to that. It felt as if, every day, her work became a more complex and confusing mental jigsaw. If the question in Operation Sigma had been simple enough – was the original bugged conversation on Igor’s super-yacht genuine or faked for their benefit? – the same could be said of this latest twist: was Igor and Mikhail’s story true and their offer real?

But how to be sure of the answer? If what everyone sought was incontrovertible proof, all they ever had to fall back on was instinct – and primarily her instinct at that.

But it didn’t waver. Not truly. She knew politicians were always going to want the comfort of proof, but intelligence rarely worked like that. Sometimes rock-solid instinct was all you were going to get. Thank God for Sir Alan. He, at least, understood this implicitly.

‘I’d better call Downing Street,’ he said. ‘I’ll place a small bet they’ll spin this as evidence of the wisdom of caution – which is, of course, another aspect we should think about carefully.’ He ended the call.

‘You look like you’ve seen a ghost,’ Julie said.

Kate mumbled a vague reply. She turned away and walked up to her office. Suzy was waiting for her. She had a way of addressing people – both hands on hips, her angular body tilted slightly forward – that they certainly didn’t teach in Charm School. ‘The original Operation Sigma case file is still locked,’ she said.

Kate was tempted to reply, Of course it bloody is, but her brain was clouded by the conversation she’d just had with the chief, and its implications. ‘Yes,’ she said.

‘Why?’

‘Because we couldn’t prove the prime minister was working for the Russians, we had exhausted all potential leads, and, under the circumstances, it felt like locking away the allegations was the responsible thing to do.’ Kate almost added that it had not been her choice, but decided against it. She sensed that, with Suzy, every word was likely to be taken down and used in evidence against her.