The SVR? Its supposedly departing head, Vasily Durov? Igor Borodin and his son Mikhail?

The Russian Mafia?

But why not kill her, too? Why wait for her to leave and kill only him?

Somebody passed outside. Christ, what about the guards? What would they do when they discovered this? She’d be arrested, tried, sent to prison for more than a lifetime . . .

She had to get out of here. Now. Her DNA was everywhere – all over every inch of him and the carriage. No attempt at a clean-up was likely to do any more than deepen a state of suspicion as to her true intent. She took her bag from the rack, opened the door and looked up and down the corridor. It was dark, save for the night-lights and the moonlight reflected off the snowy landscape beyond.

She pulled the door firmly shut behind her and walked away down the carriage. She went through the connecting door to the next. It was quiet, but for the rattle and hum of the wheels on the tracks below. At the end of the corridor, she heard two attendants chatting quietly in their galley, but she flitted past them into another carriage. And then another, until she could progress no further. She put her bag down by the door and simply waited.

She pulled down the window a fraction and lit a cigarette. She smoked it as she looked out at the endless succession of snow-covered pines, fields and houses.

Kate waited for the consequences of the terrible scene she had witnessed to take their natural turn, but the strangest part of it all was that nothing whatsoever happened. The train did not stop suddenly at a small station in the middle of nowhere. No police stepped on board. No guards came running.

After an hour, or perhaps two, a guard came to his galley at the far end of the train. He caught sight of her and put his head around the side to ask if she was all right. She shrugged, pretending not to understand the language, and he left her in peace.

In the dead of night, nothing stirred. She smoked cigarette after cigarette, then closed the window and sat on the floor, resting her head uncomfortably on the wall of the carriage.

Dawn broke. The train clattered on through the grubby outskirts of the Russian capital and eventually glided to a halt inside Leningradsky station.

Kate was the first off. She did not look over her shoulder, because they knew where to find her if that was their intention, though she did have the clarity of mind to reflect on how little idea she really had of who ‘they’ were. Tired, distraught, shocked as she might be, nothing made any sense at all.