There could be only two candidates for his murder, surely: either it was the GRU shutting down a renegade officer who’d had no motive but to assist the woman he’d always loved. (What was it he had told her? You think, after all these years, that your husband was the Russian’s state’s only asset at the heart of British Intelligence?) Or it was Mikhail, Igor and those close to them trying to protect their exit strategy.

She turned it over and over in her mind. Was it Sergei they had been watching, or had they been tipped off that she was on her way and followed her from the moment she’d crossed the border?

Kate flopped over, so that she was face down; the way she traditionally slept. She could smell her husband in the sheets and the sensation was comforting.

They’d known she was coming. That was what she kept returning to. And they’d killed Sergei before he could give her a full account of Moscow’s men – or women – in London.

But he might already have done so, in which case why not kill her, too?

Because to murder her would cause them problems. There would be an international outcry: a British civil servant killed on a Russian train. Too much trouble, perhaps. Better to send her back confused and disoriented.

They were protecting someone. They’d been tipped off she was coming and they’d closed down her source.

That much seemed clear to her. Concentrate, she told herself, on the pieces you can see and understand, not the many parts of the jigsaw you can’t know and may never know.

Kate shut her eyes. She tried to think of something else. She breathed in hard again and attempted to transport herself back to the carefree days of her life with Stuart. She turned from one image of him to the next: walking along Constantine Bay in Cornwall the summer before last; Stuart laughing his head off, roaring drunk, on the night the picture beside her had been taken in the Dolomites.

After a while, she noticed the sheet beneath her was damp with tears.

She pressed her face down harder, trying to clear her mind, as she waited and waited for sleep to come.

In the end, the chemicals did the trick, but, as ever these days, not for long. When she awoke, the clock on the table beside her told her it was just after midday. Kate got up and pulled back the curtain to reveal shards of sunlight glinting off the rows of concrete tower blocks. It was bright. The BMW had disappeared. There was a white van further along, parked with its engine running. Perhaps they planned to kidnap her.

She sat on the bed. She felt grim, but was gripped now with at least one clear thought: she had to get out of here, right away, by any means possible.