‘The foreign secretary was reluctant to accept Igor Borodin’s defection at face value,’ Grove went on. ‘She wanted more evidence. That was why you went to St Petersburg and then Moscow in search of Sergei?’

‘Yes.’

‘He confirmed your supposition at the time, that Igor was losing the power struggle and needed to get out of Russia?’

‘Yes.’

‘And you believed Igor Borodin killed Sergei to prevent any potential interference with his planned defection?’

‘Not immediately, but I came to that conclusion shortly after it went wrong.’

‘Why?’

‘It was the only explanation that made sense to me.’

‘At the time.’

‘At the time, yes.’ Kate would dearly have loved to find a way to make Igor pay for Sergei’s murder. But it was too late for that.

Grove turned another page. ‘When Mr Borodin told you on the tarmac at Tbilisi airport that the prime minister was definitely working for Moscow and that Ian had unwittingly passed on information to our enemies by keeping him informed at every turn, you believed that too at the time?’

‘I did, yes.’

‘So, in short, when you took off from Tbilisi, you were firmly of the view that the PM was a traitor and that Ian Granger was, at best, an indiscreet and ambitious fool who had unwittingly assisted him.’

Kate glanced out of the window at Ian, who was pacing the corridor. ‘Yes.’

‘You have not heard anything from Igor Borodin since that conversation on the tarmac?’

‘No one has. Not us, not GCHQ. He and his family have vanished off the face of the earth.’

‘Where did you think they had gone?’

‘I assumed that, if the GRU had won the power struggle and he was caught in the act of defecting, he and his family were probably in a Siberian gulag or dead already.’

‘I see,’ Shirley Grove said, without emotion. She turned over another page and cleared her throat as she approached the climax of this charade. ‘And yet you now say that everything you once believed in relation to this case was wrong?’

‘That’s correct.’ Kate met her deputy’s flinty glaze. How slow she had been to realize that Suzy’s true purpose – as instructed by Grove and her master, the prime minister, no doubt, and, of course, aided and abetted by Ian – had not in fact been to open up the Operation Sigma file but to find the means and the method to ensure it remained closed. For ever.