Knox reached the entrance to Blackfriars station at the northern end of the bridge. He passed two paper sellers trying to get rid of their last copies of the Evening Standard and the Evening News. Both were leading with stories about the rapid build-up of American military in West Berlin. Knox slowed down to read the headlines, and see if the man following him would carry on past him or linger on the bridge. He headed straight into the station and, after a moment, so did Knox.

He made his way down to the tube and stopped in the middle of the platform, already anticipating his change at Charing Cross to the Northern Line, which would take him up to Highgate.

The man in the bomber was further down the platform, keeping enough distance between him and Knox to put them in different carriages when the tube arrived. If he was following Knox, he was being smart about it. And if this was another round in Knox’s game with Peterson, he’d sent a much better player. Out of the corner of his eye, Knox watched him lean against the wall next to one of the station roundels. With the sudden context he realised the man was a giant. Six foot five, at least, and built like the side of a house.

A bright red Circle Line train pulled into the station, screeching loudly as its worn brakes grabbed at the tracks. The doors opened with a judder and a few commuters got off. Knox hung back on the platform. So did the giant. At the last moment, Knox hopped through the closing doors. So did the giant.

The train lumbered its way to the next stop, speeding up only to slow down again over and over, as if it wasn’t sure what lay ahead in the dark tunnel. Knox resisted the urge to look at the other passengers. He wanted to be as inconspicuous as possible, and the easiest way to stand out and draw attention in London was to make eye contact with people on the tube.

Temple station was empty. No one got off, and no one got on. Knox snatched a glance through the open doors before they slid shut again, but couldn’t see any movement in the next carriage.

Charing Cross was next. This time, as soon as the doors opened, Knox burst off the train and sprinted down the stairs his carriage had lined up perfectly with. He didn’t look back, so he didn’t see the giant pushing his way down the platform and into the tunnel, trying to catch up with him.